Style Guide for Python Code

PEP 8 - Style Guide for Python Code

PEP:	8
Title:	Style Guide for Python Code
Author:	Guido van Rossum <guido at python.org>, Barry Warsaw <barry at python.org>, Nick Coghlan <ncoghlan at gmail.com>
Status:	Active
Type:	Process
Created:	05-Jul-2001
Post-History:	05-Jul-2001, 01-Aug-2013

---

Contents

• Introduction
• A Foolish Consistency is the Hobgoblin of Little Minds
• Code lay-out
  • Indentation
  • Tabs or Spaces?
  • Maximum Line Length
  • Blank Lines
  • Source File Encoding
  • Imports
• String Quotes
• Whitespace in Expressions and Statements
  • Pet Peeves
  • Other Recommendations
• Comments
  • Block Comments
  • Inline Comments
  • Documentation Strings
• Version Bookkeeping
• Naming Conventions
  • Overriding Principle
  • Descriptive: Naming Styles
  • Prescriptive: Naming Conventions
    • Names to Avoid
    • Package and Module Names
    • Class Names
    • Exception Names
    • Global Variable Names
    • Function Names
    • Function and method arguments
    • Method Names and Instance Variables
    • Constants
    • Designing for inheritance
  • Public and internal interfaces
• Programming Recommendations
• References
• Copyright

Introduction

This document gives coding conventions for the Python code comprising the standard library in the main Python distribution. Please see the companion informational PEP describing style guidelines for the C code in the C implementation of Python [1] .
This document and PEP 257 (Docstring Conventions) were adapted from Guido's original Python Style Guide essay, with some additions from Barry's style guide [2] .
This style guide evolves over time as additional conventions are identified and past conventions are rendered obsolete by changes in the language itself.
Many projects have their own coding style guidelines. In the event of any conflicts, such project-specific guides take precedence for that project.

A Foolish Consistency is the Hobgoblin of Little Minds

One of Guido's key insights is that code is read much more often than it is written. The guidelines provided here are intended to improve the readability of code and make it consistent across the wide spectrum of Python code. As PEP 20 says, "Readability counts".
A style guide is about consistency. Consistency with this style guide is important. Consistency within a project is more important. Consistency within one module or function is most important.
But most importantly: know when to be inconsistent -- sometimes the style guide just doesn't apply. When in doubt, use your best judgment. Look at other examples and decide what looks best. And don't hesitate to ask!
In particular: do not break backwards compatibility just to comply with this PEP!
Some other good reasons to ignore a particular guideline:

1. When applying the guideline would make the code less readable, even for someone who is used to reading code that follows this PEP.
2. To be consistent with surrounding code that also breaks it (maybe for historic reasons) -- although this is also an opportunity to clean up someone else's mess (in true XP style).
3. Because the code in question predates the introduction of the guideline and there is no other reason to be modifying that code.
4. When the code needs to remain compatible with older versions of Python that don't support the feature recommended by the style guide.

Code lay-out

Indentation

Use 4 spaces per indentation level.
Continuation lines should align wrapped elements either vertically using Python's implicit line joining inside parentheses, brackets and braces, or using a hanging indent [5] . When using a hanging indent the following considerations should be applied; there should be no arguments on the first line and further indentation should be used to clearly distinguish itself as a continuation line.
Yes:

# Aligned with opening delimiter.
foo = long_function_name(var_one, var_two,
                         var_three, var_four)

# More indentation included to distinguish this from the rest.
def long_function_name(
        var_one, var_two, var_three,
        var_four):
    print(var_one)

# Hanging indents should add a level.
foo = long_function_name(
    var_one, var_two,
    var_three, var_four)

No:

# Arguments on first line forbidden when not using vertical alignment.
foo = long_function_name(var_one, var_two,
    var_three, var_four)

# Further indentation required as indentation is not distinguishable.
def long_function_name(
    var_one, var_two, var_three,
    var_four):
    print(var_one)

The 4-space rule is optional for continuation lines.
Optional:

# Hanging indents *may* be indented to other than 4 spaces.
foo = long_function_name(
  var_one, var_two,
  var_three, var_four)

When the conditional part of an if -statement is long enough to require that it be written across multiple lines, it's worth noting that the combination of a two character keyword (i.e. if ), plus a single space, plus an opening parenthesis creates a natural 4-space indent for the subsequent lines of the multiline conditional. This can produce a visual conflict with the indented suite of code nested inside the if -statement, which would also naturally be indented to 4 spaces. This PEP takes no explicit position on how (or whether) to further visually distinguish such conditional lines from the nested suite inside the if -statement. Acceptable options in this situation include, but are not limited to:

# No extra indentation.
if (this_is_one_thing and
    that_is_another_thing):
    do_something()

# Add a comment, which will provide some distinction in editors
# supporting syntax highlighting.
if (this_is_one_thing and
    that_is_another_thing):
    # Since both conditions are true, we can frobnicate.
    do_something()

# Add some extra indentation on the conditional continuation line.
if (this_is_one_thing
        and that_is_another_thing):
    do_something()

The closing brace/bracket/parenthesis on multi-line constructs may either line up under the first non-whitespace character of the last line of list, as in:

my_list = [
    1, 2, 3,
    4, 5, 6,
    ]
result = some_function_that_takes_arguments(
    'a', 'b', 'c',
    'd', 'e', 'f',
    )

or it may be lined up under the first character of the line that starts the multi-line construct, as in:

my_list = [
    1, 2, 3,
    4, 5, 6,
]
result = some_function_that_takes_arguments(
    'a', 'b', 'c',
    'd', 'e', 'f',
)

Tabs or Spaces?

Spaces are the preferred indentation method.
Tabs should be used solely to remain consistent with code that is already indented with tabs.
Python 3 disallows mixing the use of tabs and spaces for indentation.
Python 2 code indented with a mixture of tabs and spaces should be converted to using spaces exclusively.
When invoking the Python 2 command line interpreter with the -t option, it issues warnings about code that illegally mixes tabs and spaces. When using -tt these warnings become errors. These options are highly recommended!

Maximum Line Length

Limit all lines to a maximum of 79 characters.
For flowing long blocks of text with fewer structural restrictions (docstrings or comments), the line length should be limited to 72 characters.
Limiting the required editor window width makes it possible to have several files open side-by-side, and works well when using code review tools that present the two versions in adjacent columns.
The default wrapping in most tools disrupts the visual structure of the code, making it more difficult to understand. The limits are chosen to avoid wrapping in editors with the window width set to 80, even if the tool places a marker glyph in the final column when wrapping lines. Some web based tools may not offer dynamic line wrapping at all.
Some teams strongly prefer a longer line length. For code maintained exclusively or primarily by a team that can reach agreement on this issue, it is okay to increase the nominal line length from 80 to 100 characters (effectively increasing the maximum length to 99 characters), provided that comments and docstrings are still wrapped at 72 characters.
The Python standard library is conservative and requires limiting lines to 79 characters (and docstrings/comments to 72).
The preferred way of wrapping long lines is by using Python's implied line continuation inside parentheses, brackets and braces. Long lines can be broken over multiple lines by wrapping expressions in parentheses. These should be used in preference to using a backslash for line continuation.
Backslashes may still be appropriate at times. For example, long, multiple with -statements cannot use implicit continuation, so backslashes are acceptable:

with open('/path/to/some/file/you/want/to/read') as file_1, \
     open('/path/to/some/file/being/written', 'w') as file_2:
    file_2.write(file_1.read())

(See the previous discussion on multiline if-statements for further thoughts on the indentation of such multiline with -statements.)
Another such case is with assert statements.
Make sure to indent the continued line appropriately. The preferred place to break around a binary operator is after the operator, not before it. Some examples:

class Rectangle(Blob):

    def __init__(self, width, height,
                 color='black', emphasis=None, highlight=0):
        if (width == 0 and height == 0 and
                color == 'red' and emphasis == 'strong' or
                highlight > 100):
            raise ValueError("sorry, you lose")
        if width == 0 and height == 0 and (color == 'red' or
                                           emphasis is None):
            raise ValueError("I don't think so -- values are %s, %s" %
                             (width, height))
        Blob.__init__(self, width, height,
                      color, emphasis, highlight)

Blank Lines

Separate top-level function and class definitions with two blank lines.
Method definitions inside a class are separated by a single blank line.
Extra blank lines may be used (sparingly) to separate groups of related functions. Blank lines may be omitted between a bunch of related one-liners (e.g. a set of dummy implementations).
Use blank lines in functions, sparingly, to indicate logical sections.
Python accepts the control-L (i.e. ^L) form feed character as whitespace; Many tools treat these characters as page separators, so you may use them to separate pages of related sections of your file. Note, some editors and web-based code viewers may not recognize control-L as a form feed and will show another glyph in its place.

Source File Encoding

Code in the core Python distribution should always use UTF-8 (or ASCII in Python 2).
Files using ASCII (in Python 2) or UTF-8 (in Python 3) should not have an encoding declaration.
In the standard library, non-default encodings should be used only for test purposes or when a comment or docstring needs to mention an author name that contains non-ASCII characters; otherwise, using \x , \u , \U , or \N escapes is the preferred way to include non-ASCII data in string literals.
For Python 3.0 and beyond, the following policy is prescribed for the standard library (see PEP 3131 ): All identifiers in the Python standard library MUST use ASCII-only identifiers, and SHOULD use English words wherever feasible (in many cases, abbreviations and technical terms are used which aren't English). In addition, string literals and comments must also be in ASCII. The only exceptions are (a) test cases testing the non-ASCII features, and (b) names of authors. Authors whose names are not based on the latin alphabet MUST provide a latin transliteration of their names.
Open source projects with a global audience are encouraged to adopt a similar policy.

Imports

• Imports should usually be on separate lines, e.g.:

  Yes: import os
       import sys
  
  No:  import sys, os
  

  It's okay to say this though:
  

  from subprocess import Popen, PIPE
  

• Imports are always put at the top of the file, just after any module comments and docstrings, and before module globals and constants.
  Imports should be grouped in the following order:
  
  1. standard library imports
  2. related third party imports
  3. local application/library specific imports
  You should put a blank line between each group of imports.
  Put any relevant __all__ specification after the imports.
  
• Absolute imports are recommended, as they are usually more readable and tend to be better behaved (or at least give better error messages) if the import system is incorrectly configured (such as when a directory inside a package ends up on sys.path ):

  import mypkg.sibling
  from mypkg import sibling
  from mypkg.sibling import example
  

  However, explicit relative imports are an acceptable alternative to absolute imports, especially when dealing with complex package layouts where using absolute imports would be unnecessarily verbose:
  

  from . import sibling
  from .sibling import example
  

  Standard library code should avoid complex package layouts and always use absolute imports.
  Implicit relative imports should never be used and have been removed in Python 3.
  
• When importing a class from a class-containing module, it's usually okay to spell this:

  from myclass import MyClass
  from foo.bar.yourclass import YourClass
  

  If this spelling causes local name clashes, then spell them
  

  import myclass
  import foo.bar.yourclass
  

  and use "myclass.MyClass" and "foo.bar.yourclass.YourClass".
  
• Wildcard imports ( from <module> import * ) should be avoided, as they make it unclear which names are present in the namespace, confusing both readers and many automated tools. There is one defensible use case for a wildcard import, which is to republish an internal interface as part of a public API (for example, overwriting a pure Python implementation of an interface with the definitions from an optional accelerator module and exactly which definitions will be overwritten isn't known in advance).
  When republishing names this way, the guidelines below regarding public and internal interfaces still apply.
  

String Quotes

In Python, single-quoted strings and double-quoted strings are the same. This PEP does not make a recommendation for this. Pick a rule and stick to it. When a string contains single or double quote characters, however, use the other one to avoid backslashes in the string. It improves readability.
For triple-quoted strings, always use double quote characters to be consistent with the docstring convention in PEP 257 .

Whitespace in Expressions and Statements

Pet Peeves

Avoid extraneous whitespace in the following situations:

• Immediately inside parentheses, brackets or braces.

  Yes: spam(ham[1], {eggs: 2})
  No:  spam( ham[ 1 ], { eggs: 2 } )
  

• Immediately before a comma, semicolon, or colon:

  Yes: if x == 4: print x, y; x, y = y, x
  No:  if x == 4 : print x , y ; x , y = y , x
  

• However, in a slice the colon acts like a binary operator, and should have equal amounts on either side (treating it as the operator with the lowest priority). In an extended slice, both colons must have the same amount of spacing applied. Exception: when a slice parameter is omitted, the space is omitted.
  Yes:
  

  ham[1:9], ham[1:9:3], ham[:9:3], ham[1::3], ham[1:9:]
  ham[lower:upper], ham[lower:upper:], ham[lower::step]
  ham[lower+offset : upper+offset]
  ham[: upper_fn(x) : step_fn(x)], ham[:: step_fn(x)]
  ham[lower + offset : upper + offset]
  

  No:
  

  ham[lower + offset:upper + offset]
  ham[1: 9], ham[1 :9], ham[1:9 :3]
  ham[lower : : upper]
  ham[ : upper]
  

• Immediately before the open parenthesis that starts the argument list of a function call:

  Yes: spam(1)
  No:  spam (1)
  

• Immediately before the open parenthesis that starts an indexing or slicing:

  Yes: dct['key'] = lst[index]
  No:  dct ['key'] = lst [index]
  

• More than one space around an assignment (or other) operator to align it with another.
  Yes:
  

  x = 1
  y = 2
  long_variable = 3
  

  No:
  

  x             = 1
  y             = 2
  long_variable = 3
  

Other Recommendations

• Always surround these binary operators with a single space on either side: assignment ( = ), augmented assignment ( += , -= etc.), comparisons ( == , < , > , != , <> , <= , >= , in , not in , is , is not ), Booleans ( and , or , not ).
• If operators with different priorities are used, consider adding whitespace around the operators with the lowest priority(ies). Use your own judgment; however, never use more than one space, and always have the same amount of whitespace on both sides of a binary operator.
  Yes:
  

  i = i + 1
  submitted += 1
  x = x*2 - 1
  hypot2 = x*x + y*y
  c = (a+b) * (a-b)
  

  No:
  

  i=i+1
  submitted +=1
  x = x * 2 - 1
  hypot2 = x * x + y * y
  c = (a + b) * (a - b)
  

• Don't use spaces around the = sign when used to indicate a keyword argument or a default parameter value.
  Yes:
  

  def complex(real, imag=0.0):
      return magic(r=real, i=imag)
  

  No:
  

  def complex(real, imag = 0.0):
      return magic(r = real, i = imag)
  

• Do use spaces around the = sign of an annotated function definition. Additionally, use a single space after the : , as well as a single space on either side of the -> sign representing an annotated return value.
  Yes:
  

  def munge(input: AnyStr):
  def munge(sep: AnyStr = None):
  def munge() -> AnyStr:
  def munge(input: AnyStr, sep: AnyStr = None, limit=1000):
  

  No:
  

  def munge(input: AnyStr=None):
  def munge(input:AnyStr):
  def munge(input: AnyStr)->PosInt:
  

• Compound statements (multiple statements on the same line) are generally discouraged.
  Yes:
  

  if foo == 'blah':
      do_blah_thing()
  do_one()
  do_two()
  do_three()
  

  Rather not:
  

  if foo == 'blah': do_blah_thing()
  do_one(); do_two(); do_three()
  

• While sometimes it's okay to put an if/for/while with a small body on the same line, never do this for multi-clause statements. Also avoid folding such long lines!
  Rather not:
  

  if foo == 'blah': do_blah_thing()
  for x in lst: total += x
  while t < 10: t = delay()
  

  Definitely not:
  

  if foo == 'blah': do_blah_thing()
  else: do_non_blah_thing()
  
  try: something()
  finally: cleanup()
  
  do_one(); do_two(); do_three(long, argument,
                               list, like, this)
  
  if foo == 'blah': one(); two(); three()
  

Version Bookkeeping

If you have to have Subversion, CVS, or RCS crud in your source file, do it as follows.

__version__ = "$Revision$"
# $Source$

These lines should be included after the module's docstring, before any other code, separated by a blank line above and below.

Naming Conventions

The naming conventions of Python's library are a bit of a mess, so we'll never get this completely consistent -- nevertheless, here are the currently recommended naming standards. New modules and packages (including third party frameworks) should be written to these standards, but where an existing library has a different style, internal consistency is preferred.

Overriding Principle

Names that are visible to the user as public parts of the API should follow conventions that reflect usage rather than implementation.

Descriptive: Naming Styles

There are a lot of different naming styles. It helps to be able to recognize what naming style is being used, independently from what they are used for.
The following naming styles are commonly distinguished:

• b (single lowercase letter)
• B (single uppercase letter)
• lowercase
• lower_case_with_underscores
• UPPERCASE
• UPPER_CASE_WITH_UNDERSCORES
• CapitalizedWords (or CapWords, or CamelCase -- so named because of the bumpy look of its letters [3] ). This is also sometimes known as StudlyCaps.
  Note: When using abbreviations in CapWords, capitalize all the letters of the abbreviation. Thus HTTPServerError is better than HttpServerError.
  
• mixedCase (differs from CapitalizedWords by initial lowercase character!)
• Capitalized_Words_With_Underscores (ugly!)

There's also the style of using a short unique prefix to group related names together. This is not used much in Python, but it is mentioned for completeness. For example, the os.stat() function returns a tuple whose items traditionally have names like st_mode , st_size , st_mtime and so on. (This is done to emphasize the correspondence with the fields of the POSIX system call struct, which helps programmers familiar with that.)
The X11 library uses a leading X for all its public functions. In Python, this style is generally deemed unnecessary because attribute and method names are prefixed with an object, and function names are prefixed with a module name.
In addition, the following special forms using leading or trailing underscores are recognized (these can generally be combined with any case convention):

• _single_leading_underscore : weak "internal use" indicator. E.g. from M import * does not import objects whose name starts with an underscore.
• single_trailing_underscore_ : used by convention to avoid conflicts with Python keyword, e.g.

  Tkinter.Toplevel(master, class_='ClassName')
  

• __double_leading_underscore : when naming a class attribute, invokes name mangling (inside class FooBar, __boo becomes _FooBar__boo ; see below).
• __double_leading_and_trailing_underscore__ : "magic" objects or attributes that live in user-controlled namespaces. E.g. __init__ , __import__ or __file__ . Never invent such names; only use them as documented.

Prescriptive: Naming Conventions

Names to Avoid

Never use the characters 'l' (lowercase letter el), 'O' (uppercase letter oh), or 'I' (uppercase letter eye) as single character variable names.
In some fonts, these characters are indistinguishable from the numerals one and zero. When tempted to use 'l', use 'L' instead.

Package and Module Names

Modules should have short, all-lowercase names. Underscores can be used in the module name if it improves readability. Python packages should also have short, all-lowercase names, although the use of underscores is discouraged.
Since module names are mapped to file names, and some file systems are case insensitive and truncate long names, it is important that module names be chosen to be fairly short -- this won't be a problem on Unix, but it may be a problem when the code is transported to older Mac or Windows versions, or DOS.
When an extension module written in C or C++ has an accompanying Python module that provides a higher level (e.g. more object oriented) interface, the C/C++ module has a leading underscore (e.g. _socket ).

Class Names

Class names should normally use the CapWords convention.
The naming convention for functions may be used instead in cases where the interface is documented and used primarily as a callable.
Note that there is a separate convention for builtin names: most builtin names are single words (or two words run together), with the CapWords convention used only for exception names and builtin constants.

Exception Names

Because exceptions should be classes, the class naming convention applies here. However, you should use the suffix "Error" on your exception names (if the exception actually is an error).

Global Variable Names

(Let's hope that these variables are meant for use inside one module only.) The conventions are about the same as those for functions.
Modules that are designed for use via from M import * should use the __all__ mechanism to prevent exporting globals, or use the older convention of prefixing such globals with an underscore (which you might want to do to indicate these globals are "module non-public").

Function Names

Function names should be lowercase, with words separated by underscores as necessary to improve readability.
mixedCase is allowed only in contexts where that's already the prevailing style (e.g. threading.py), to retain backwards compatibility.

Function and method arguments

Always use self for the first argument to instance methods.
Always use cls for the first argument to class methods.
If a function argument's name clashes with a reserved keyword, it is generally better to append a single trailing underscore rather than use an abbreviation or spelling corruption. Thus class_ is better than clss . (Perhaps better is to avoid such clashes by using a synonym.)

Method Names and Instance Variables

Use the function naming rules: lowercase with words separated by underscores as necessary to improve readability.
Use one leading underscore only for non-public methods and instance variables.
To avoid name clashes with subclasses, use two leading underscores to invoke Python's name mangling rules.
Python mangles these names with the class name: if class Foo has an attribute named __a , it cannot be accessed by Foo.__a . (An insistent user could still gain access by calling Foo._Foo__a .) Generally, double leading underscores should be used only to avoid name conflicts with attributes in classes designed to be subclassed.
Note: there is some controversy about the use of __names (see below).

Constants

Constants are usually defined on a module level and written in all capital letters with underscores separating words. Examples include MAX_OVERFLOW and TOTAL .

Designing for inheritance

Always decide whether a class's methods and instance variables (collectively: "attributes") should be public or non-public. If in doubt, choose non-public; it's easier to make it public later than to make a public attribute non-public.
Public attributes are those that you expect unrelated clients of your class to use, with your commitment to avoid backward incompatible changes. Non-public attributes are those that are not intended to be used by third parties; you make no guarantees that non-public attributes won't change or even be removed.
We don't use the term "private" here, since no attribute is really private in Python (without a generally unnecessary amount of work).
Another category of attributes are those that are part of the "subclass API" (often called "protected" in other languages). Some classes are designed to be inherited from, either to extend or modify aspects of the class's behavior. When designing such a class, take care to make explicit decisions about which attributes are public, which are part of the subclass API, and which are truly only to be used by your base class.
With this in mind, here are the Pythonic guidelines:

• Public attributes should have no leading underscores.
• If your public attribute name collides with a reserved keyword, append a single trailing underscore to your attribute name. This is preferable to an abbreviation or corrupted spelling. (However, notwithstanding this rule, 'cls' is the preferred spelling for any variable or argument which is known to be a class, especially the first argument to a class method.)
  Note 1: See the argument name recommendation above for class methods.
  
• For simple public data attributes, it is best to expose just the attribute name, without complicated accessor/mutator methods. Keep in mind that Python provides an easy path to future enhancement, should you find that a simple data attribute needs to grow functional behavior. In that case, use properties to hide functional implementation behind simple data attribute access syntax.
  Note 1: Properties only work on new-style classes.
  Note 2: Try to keep the functional behavior side-effect free, although side-effects such as caching are generally fine.
  Note 3: Avoid using properties for computationally expensive operations; the attribute notation makes the caller believe that access is (relatively) cheap.
  
• If your class is intended to be subclassed, and you have attributes that you do not want subclasses to use, consider naming them with double leading underscores and no trailing underscores. This invokes Python's name mangling algorithm, where the name of the class is mangled into the attribute name. This helps avoid attribute name collisions should subclasses inadvertently contain attributes with the same name.
  Note 1: Note that only the simple class name is used in the mangled name, so if a subclass chooses both the same class name and attribute name, you can still get name collisions.
  Note 2: Name mangling can make certain uses, such as debugging and __getattr__() , less convenient. However the name mangling algorithm is well documented and easy to perform manually.
  Note 3: Not everyone likes name mangling. Try to balance the need to avoid accidental name clashes with potential use by advanced callers.
  

Public and internal interfaces

Any backwards compatibility guarantees apply only to public interfaces. Accordingly, it is important that users be able to clearly distinguish between public and internal interfaces.
Documented interfaces are considered public, unless the documentation explicitly declares them to be provisional or internal interfaces exempt from the usual backwards compatibility guarantees. All undocumented interfaces should be assumed to be internal.
To better support introspection, modules should explicitly declare the names in their public API using the __all__ attribute. Setting __all__ to an empty list indicates that the module has no public API.
Even with __all__ set appropriately, internal interfaces (packages, modules, classes, functions, attributes or other names) should still be prefixed with a single leading underscore.
An interface is also considered internal if any containing namespace (package, module or class) is considered internal.
Imported names should always be considered an implementation detail. Other modules must not rely on indirect access to such imported names unless they are an explicitly documented part of the containing module's API, such as os.path or a package's __init__ module that exposes functionality from submodules.

Programming Recommendations

• Code should be written in a way that does not disadvantage other implementations of Python (PyPy, Jython, IronPython, Cython, Psyco, and such).
  For example, do not rely on CPython's efficient implementation of in-place string concatenation for statements in the form a += b or a = a + b . This optimization is fragile even in CPython (it only works for some types) and isn't present at all in implementations that don't use refcounting. In performance sensitive parts of the library, the ''.join() form should be used instead. This will ensure that concatenation occurs in linear time across various implementations.
  
• Comparisons to singletons like None should always be done with is or is not , never the equality operators.
  Also, beware of writing if x when you really mean if x is not None -- e.g. when testing whether a variable or argument that defaults to None was set to some other value. The other value might have a type (such as a container) that could be false in a boolean context!
  
• Use is not operator rather than not ... is . While both expressions are functionally identical, the former is more readable and preferred.
  Yes:
  

  if foo is not None:
  

  No:
  

  if not foo is None:
  

• When implementing ordering operations with rich comparisons, it is best to implement all six operations ( __eq__ , __ne__ , __lt__ , __le__ , __gt__ , __ge__ ) rather than relying on other code to only exercise a particular comparison.
  To minimize the effort involved, the functools.total_ordering() decorator provides a tool to generate missing comparison methods.
  PEP 207 indicates that reflexivity rules are assumed by Python. Thus, the interpreter may swap y > x with x < y , y >= x with x <= y , and may swap the arguments of x == y and x != y . The sort() and min() operations are guaranteed to use the < operator and the max() function uses the > operator. However, it is best to implement all six operations so that confusion doesn't arise in other contexts.
  
• Always use a def statement instead of an assignment statement that binds a lambda expression directly to an identifier.
  Yes:
  

  def f(x): return 2*x
  

  No:
  

  f = lambda x: 2*x
  

  The first form means that the name of the resulting function object is specifically 'f' instead of the generic '<lambda>'. This is more useful for tracebacks and string representations in general. The use of the assignment statement eliminates the sole benefit a lambda expression can offer over an explicit def statement (i.e. that it can be embedded inside a larger expression)
  
• Derive exceptions from Exception rather than BaseException . Direct inheritance from BaseException is reserved for exceptions where catching them is almost always the wrong thing to do.
  Design exception hierarchies based on the distinctions that code catching the exceptions is likely to need, rather than the locations where the exceptions are raised. Aim to answer the question "What went wrong?" programmatically, rather than only stating that "A problem occurred" (see PEP 3151 for an example of this lesson being learned for the builtin exception hierarchy)
  Class naming conventions apply here, although you should add the suffix "Error" to your exception classes if the exception is an error. Non-error exceptions that are used for non-local flow control or other forms of signaling need no special suffix.
  
• Use exception chaining appropriately. In Python 3, "raise X from Y" should be used to indicate explicit replacement without losing the original traceback.
  When deliberately replacing an inner exception (using "raise X" in Python 2 or "raise X from None" in Python 3.3+), ensure that relevant details are transferred to the new exception (such as preserving the attribute name when converting KeyError to AttributeError, or embedding the text of the original exception in the new exception message).
  
• When raising an exception in Python 2, use raise ValueError('message') instead of the older form raise ValueError, 'message' .
  The latter form is not legal Python 3 syntax.
  The paren-using form also means that when the exception arguments are long or include string formatting, you don't need to use line continuation characters thanks to the containing parentheses.
  
• When catching exceptions, mention specific exceptions whenever possible instead of using a bare except: clause.
  For example, use:
  

  try:
      import platform_specific_module
  except ImportError:
      platform_specific_module = None
  

  A bare except: clause will catch SystemExit and KeyboardInterrupt exceptions, making it harder to interrupt a program with Control-C, and can disguise other problems. If you want to catch all exceptions that signal program errors, use except Exception: (bare except is equivalent to except BaseException: ).
  A good rule of thumb is to limit use of bare 'except' clauses to two cases:
  
  1. If the exception handler will be printing out or logging the traceback; at least the user will be aware that an error has occurred.
  2. If the code needs to do some cleanup work, but then lets the exception propagate upwards with raise . try...finally can be a better way to handle this case.
• When binding caught exceptions to a name, prefer the explicit name binding syntax added in Python 2.6:

  try:
      process_data()
  except Exception as exc:
      raise DataProcessingFailedError(str(exc))
  

  This is the only syntax supported in Python 3, and avoids the ambiguity problems associated with the older comma-based syntax.
  
• When catching operating system errors, prefer the explicit exception hierarchy introduced in Python 3.3 over introspection of errno values.
• Additionally, for all try/except clauses, limit the try clause to the absolute minimum amount of code necessary. Again, this avoids masking bugs.
  Yes:
  

  try:
      value = collection[key]
  except KeyError:
      return key_not_found(key)
  else:
      return handle_value(value)
  

  No:
  

  try:
      # Too broad!
      return handle_value(collection[key])
  except KeyError:
      # Will also catch KeyError raised by handle_value()
      return key_not_found(key)
  

• When a resource is local to a particular section of code, use a with statement to ensure it is cleaned up promptly and reliably after use. A try/finally statement is also acceptable.
• Context managers should be invoked through separate functions or methods whenever they do something other than acquire and release resources. For example:
  Yes:
  

  with conn.begin_transaction():
      do_stuff_in_transaction(conn)
  

  No:
  

  with conn:
      do_stuff_in_transaction(conn)
  

  The latter example doesn't provide any information to indicate that the __enter__ and __exit__ methods are doing something other than closing the connection after a transaction. Being explicit is important in this case.
  
• Use string methods instead of the string module.
  String methods are always much faster and share the same API with unicode strings. Override this rule if backward compatibility with Pythons older than 2.0 is required.
  
• Use ''.startswith() and ''.endswith() instead of string slicing to check for prefixes or suffixes.
  startswith() and endswith() are cleaner and less error prone. For example:
  

  Yes: if foo.startswith('bar'):
  No:  if foo[:3] == 'bar':
  

• Object type comparisons should always use isinstance() instead of comparing types directly.

  Yes: if isinstance(obj, int):
  
  No:  if type(obj) is type(1):
  

  When checking if an object is a string, keep in mind that it might be a unicode string too! In Python 2, str and unicode have a common base class, basestring, so you can do:
  

  if isinstance(obj, basestring):
  

  Note that in Python 3, unicode and basestring no longer exist (there is only str ) and a bytes object is no longer a kind of string (it is a sequence of integers instead)
  
• For sequences, (strings, lists, tuples), use the fact that empty sequences are false.

  Yes: if not seq:
       if seq:
  
  No: if len(seq)
      if not len(seq)
  

• Don't write string literals that rely on significant trailing whitespace. Such trailing whitespace is visually indistinguishable and some editors (or more recently, reindent.py) will trim them.
• Don't compare boolean values to True or False using == .

  Yes:   if greeting:
  No:    if greeting == True:
  Worse: if greeting is True:
  

• The Python standard library will not use function annotations as that would result in a premature commitment to a particular annotation style. Instead, the annotations are left for users to discover and experiment with useful annotation styles.
  It is recommended that third party experiments with annotations use an associated decorator to indicate how the annotation should be interpreted.
  Early core developer attempts to use function annotations revealed inconsistent, ad-hoc annotation styles. For example:
  
  • [str] was ambiguous as to whether it represented a list of strings or a value that could be either str or None .
  • The notation open(file:(str,bytes)) was used for a value that could be either bytes or str rather than a 2-tuple containing a str value followed by a bytes value.
  • The annotation seek(whence:int) exhibited a mix of over-specification and under-specification: int is too restrictive (anything with __index__ would be allowed) and it is not restrictive enough (only the values 0, 1, and 2 are allowed). Likewise, the annotation write(b: bytes) was also too restrictive (anything supporting the buffer protocol would be allowed).
  • Annotations such as read1(n: int=None) were self-contradictory since None is not an int . Annotations such as source_path(self, fullname:str) -> object were confusing about what the return type should be.
  • In addition to the above, annotations were inconsistent in the use of concrete types versus abstract types: int versus Integral and set/frozenset versus MutableSet/Set.
  • Some annotations in the abstract base classes were incorrect specifications. For example, set-to-set operations require other to be another instance of Set rather than just an Iterable .
  • A further issue was that annotations become part of the specification but weren't being tested.
  • In most cases, the docstrings already included the type specifications and did so with greater clarity than the function annotations. In the remaining cases, the docstrings were improved once the annotations were removed.
  • The observed function annotations were too ad-hoc and inconsistent to work with a coherent system of automatic type checking or argument validation. Leaving these annotations in the code would have made it more difficult to make changes later so that automated utilities could be supported.

Footnotes

[5]

Hanging indentation is a type-setting style where all the lines in a paragraph are indented except the first line. In the context of Python, the term is used to describe a style where the opening parenthesis of a parenthesized statement is the last non-whitespace character of the line, with subsequent lines being indented until the closing parenthesis.

References

[1]	PEP 7 , Style Guide for C Code, van Rossum

[2]	Barry's GNU Mailman style guide http://barry.warsaw.us/software/STYLEGUIDE.txt

[3]	http://www.wikipedia.com/wiki/CamelCase

[4]	PEP 8 modernisation, July 2013 http://bugs.python.org/issue18472

Copyright

This document has been placed in the public domain.

Source: https://hg.python.org/peps/file/tip/pep-0008.txt

Linux Kernel Coding Style

  Linux kernel coding style

This is a short document describing the preferred coding style for the
linux kernel.  Coding style is very personal, and I won't _force_ my
views on anybody, but this is what goes for anything that I have to be
able to maintain, and I'd prefer it for most other things too.  Please
at least consider the points made here.

First off, I'd suggest printing out a copy of the GNU coding standards,
and NOT read it.  Burn them, it's a great symbolic gesture.

Anyway, here goes:


   Chapter 1: Indentation

Tabs are 8 characters, and thus indentations are also 8 characters.
There are heretic movements that try to make indentations 4 (or even 2!)
characters deep, and that is akin to trying to define the value of PI to
be 3.

Rationale: The whole idea behind indentation is to clearly define where
a block of control starts and ends.  Especially when you've been looking
at your screen for 20 straight hours, you'll find it a lot easier to see
how the indentation works if you have large indentations.

Now, some people will claim that having 8-character indentations makes
the code move too far to the right, and makes it hard to read on a
80-character terminal screen.  The answer to that is that if you need
more than 3 levels of indentation, you're screwed anyway, and should fix
your program.

In short, 8-char indents make things easier to read, and have the added
benefit of warning you when you're nesting your functions too deep.
Heed that warning.

The preferred way to ease multiple indentation levels in a switch statement is
to align the "switch" and its subordinate "case" labels in the same column
instead of "double-indenting" the "case" labels.  E.g.:

 switch (suffix) {
 case 'G':
 case 'g':
  mem <<= 30;
  break;
 case 'M':
 case 'm':
  mem <<= 20;
  break;
 case 'K':
 case 'k':
  mem <<= 10;
  /* fall through */
 default:
  break;
 }


Don't put multiple statements on a single line unless you have
something to hide:

 if (condition) do_this;
   do_something_everytime;

Don't put multiple assignments on a single line either.  Kernel coding style
is super simple.  Avoid tricky expressions.

Outside of comments, documentation and except in Kconfig, spaces are never
used for indentation, and the above example is deliberately broken.

Get a decent editor and don't leave whitespace at the end of lines.


  Chapter 2: Breaking long lines and strings

Coding style is all about readability and maintainability using commonly
available tools.

The limit on the length of lines is 80 columns and this is a strongly
preferred limit.

Statements longer than 80 columns will be broken into sensible chunks, unless
exceeding 80 columns significantly increases readability and does not hide
information. Descendants are always substantially shorter than the parent and
are placed substantially to the right. The same applies to function headers
with a long argument list. However, never break user-visible strings such as
printk messages, because that breaks the ability to grep for them.


  Chapter 3: Placing Braces and Spaces

The other issue that always comes up in C styling is the placement of
braces.  Unlike the indent size, there are few technical reasons to
choose one placement strategy over the other, but the preferred way, as
shown to us by the prophets Kernighan and Ritchie, is to put the opening
brace last on the line, and put the closing brace first, thusly:

 if (x is true) {
  we do y
 }

This applies to all non-function statement blocks (if, switch, for,
while, do).  E.g.:

 switch (action) {
 case KOBJ_ADD:
  return "add";
 case KOBJ_REMOVE:
  return "remove";
 case KOBJ_CHANGE:
  return "change";
 default:
  return NULL;
 }

However, there is one special case, namely functions: they have the
opening brace at the beginning of the next line, thus:

 int function(int x)
 {
  body of function
 }

Heretic people all over the world have claimed that this inconsistency
is ...  well ...  inconsistent, but all right-thinking people know that
(a) K&R are _right_ and (b) K&R are right.  Besides, functions are
special anyway (you can't nest them in C).

Note that the closing brace is empty on a line of its own, _except_ in
the cases where it is followed by a continuation of the same statement,
ie a "while" in a do-statement or an "else" in an if-statement, like
this:

 do {
  body of do-loop
 } while (condition);

and

 if (x == y) {
  ..
 } else if (x > y) {
  ...
 } else {
  ....
 }

Rationale: K&R.

Also, note that this brace-placement also minimizes the number of empty
(or almost empty) lines, without any loss of readability.  Thus, as the
supply of new-lines on your screen is not a renewable resource (think
25-line terminal screens here), you have more empty lines to put
comments on.

Do not unnecessarily use braces where a single statement will do.

if (condition)
 action();

and

if (condition)
 do_this();
else
 do_that();

This does not apply if only one branch of a conditional statement is a single
statement; in the latter case use braces in both branches:

if (condition) {
 do_this();
 do_that();
} else {
 otherwise();
}

  3.1:  Spaces

Linux kernel style for use of spaces depends (mostly) on
function-versus-keyword usage.  Use a space after (most) keywords.  The
notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
somewhat like functions (and are usually used with parentheses in Linux,
although they are not required in the language, as in: "sizeof info" after
"struct fileinfo info;" is declared).

So use a space after these keywords:
 if, switch, case, for, do, while
but not with sizeof, typeof, alignof, or __attribute__.  E.g.,
 s = sizeof(struct file);

Do not add spaces around (inside) parenthesized expressions.  This example is
*bad*:

 s = sizeof( struct file );

When declaring pointer data or a function that returns a pointer type, the
preferred use of '*' is adjacent to the data name or function name and not
adjacent to the type name.  Examples:

 char *linux_banner;
 unsigned long long memparse(char *ptr, char **retptr);
 char *match_strdup(substring_t *s);

Use one space around (on each side of) most binary and ternary operators,
such as any of these:

 =  +  -  <  >  *  /  %  |  &  ^  <=  >=  ==  !=  ?  :

but no space after unary operators:
 &  *  +  -  ~  !  sizeof  typeof  alignof  __attribute__  defined

no space before the postfix increment & decrement unary operators:
 ++  --

no space after the prefix increment & decrement unary operators:
 ++  --

and no space around the '.' and "->" structure member operators.

Do not leave trailing whitespace at the ends of lines.  Some editors with
"smart" indentation will insert whitespace at the beginning of new lines as
appropriate, so you can start typing the next line of code right away.
However, some such editors do not remove the whitespace if you end up not
putting a line of code there, such as if you leave a blank line.  As a result,
you end up with lines containing trailing whitespace.

Git will warn you about patches that introduce trailing whitespace, and can
optionally strip the trailing whitespace for you; however, if applying a series
of patches, this may make later patches in the series fail by changing their
context lines.


  Chapter 4: Naming

C is a Spartan language, and so should your naming be.  Unlike Modula-2
and Pascal programmers, C programmers do not use cute names like
ThisVariableIsATemporaryCounter.  A C programmer would call that
variable "tmp", which is much easier to write, and not the least more
difficult to understand.

HOWEVER, while mixed-case names are frowned upon, descriptive names for
global variables are a must.  To call a global function "foo" is a
shooting offense.

GLOBAL variables (to be used only if you _really_ need them) need to
have descriptive names, as do global functions.  If you have a function
that counts the number of active users, you should call that
"count_active_users()" or similar, you should _not_ call it "cntusr()".

Encoding the type of a function into the name (so-called Hungarian
notation) is brain damaged - the compiler knows the types anyway and can
check those, and it only confuses the programmer.  No wonder MicroSoft
makes buggy programs.

LOCAL variable names should be short, and to the point.  If you have
some random integer loop counter, it should probably be called "i".
Calling it "loop_counter" is non-productive, if there is no chance of it
being mis-understood.  Similarly, "tmp" can be just about any type of
variable that is used to hold a temporary value.

If you are afraid to mix up your local variable names, you have another
problem, which is called the function-growth-hormone-imbalance syndrome.
See chapter 6 (Functions).


  Chapter 5: Typedefs

Please don't use things like "vps_t".

It's a _mistake_ to use typedef for structures and pointers. When you see a

 vps_t a;

in the source, what does it mean?

In contrast, if it says

 struct virtual_container *a;

you can actually tell what "a" is.

Lots of people think that typedefs "help readability". Not so. They are
useful only for:

 (a) totally opaque objects (where the typedef is actively used to _hide_
     what the object is).

     Example: "pte_t" etc. opaque objects that you can only access using
     the proper accessor functions.

     NOTE! Opaqueness and "accessor functions" are not good in themselves.
     The reason we have them for things like pte_t etc. is that there
     really is absolutely _zero_ portably accessible information there.

 (b) Clear integer types, where the abstraction _helps_ avoid confusion
     whether it is "int" or "long".

     u8/u16/u32 are perfectly fine typedefs, although they fit into
     category (d) better than here.

     NOTE! Again - there needs to be a _reason_ for this. If something is
     "unsigned long", then there's no reason to do

 typedef unsigned long myflags_t;

     but if there is a clear reason for why it under certain circumstances
     might be an "unsigned int" and under other configurations might be
     "unsigned long", then by all means go ahead and use a typedef.

 (c) when you use sparse to literally create a _new_ type for
     type-checking.

 (d) New types which are identical to standard C99 types, in certain
     exceptional circumstances.

     Although it would only take a short amount of time for the eyes and
     brain to become accustomed to the standard types like 'uint32_t',
     some people object to their use anyway.

     Therefore, the Linux-specific 'u8/u16/u32/u64' types and their
     signed equivalents which are identical to standard types are
     permitted -- although they are not mandatory in new code of your
     own.

     When editing existing code which already uses one or the other set
     of types, you should conform to the existing choices in that code.

 (e) Types safe for use in userspace.

     In certain structures which are visible to userspace, we cannot
     require C99 types and cannot use the 'u32' form above. Thus, we
     use __u32 and similar types in all structures which are shared
     with userspace.

Maybe there are other cases too, but the rule should basically be to NEVER
EVER use a typedef unless you can clearly match one of those rules.

In general, a pointer, or a struct that has elements that can reasonably
be directly accessed should _never_ be a typedef.


  Chapter 6: Functions

Functions should be short and sweet, and do just one thing.  They should
fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
as we all know), and do one thing and do that well.

The maximum length of a function is inversely proportional to the
complexity and indentation level of that function.  So, if you have a
conceptually simple function that is just one long (but simple)
case-statement, where you have to do lots of small things for a lot of
different cases, it's OK to have a longer function.

However, if you have a complex function, and you suspect that a
less-than-gifted first-year high-school student might not even
understand what the function is all about, you should adhere to the
maximum limits all the more closely.  Use helper functions with
descriptive names (you can ask the compiler to in-line them if you think
it's performance-critical, and it will probably do a better job of it
than you would have done).

Another measure of the function is the number of local variables.  They
shouldn't exceed 5-10, or you're doing something wrong.  Re-think the
function, and split it into smaller pieces.  A human brain can
generally easily keep track of about 7 different things, anything more
and it gets confused.  You know you're brilliant, but maybe you'd like
to understand what you did 2 weeks from now.

In source files, separate functions with one blank line.  If the function is
exported, the EXPORT* macro for it should follow immediately after the closing
function brace line.  E.g.:

int system_is_up(void)
{
 return system_state == SYSTEM_RUNNING;
}
EXPORT_SYMBOL(system_is_up);

In function prototypes, include parameter names with their data types.
Although this is not required by the C language, it is preferred in Linux
because it is a simple way to add valuable information for the reader.


  Chapter 7: Centralized exiting of functions

Albeit deprecated by some people, the equivalent of the goto statement is
used frequently by compilers in form of the unconditional jump instruction.

The goto statement comes in handy when a function exits from multiple
locations and some common work such as cleanup has to be done.  If there is no
cleanup needed then just return directly.

Choose label names which say what the goto does or why the goto exists.  An
example of a good name could be "out_buffer:" if the goto frees "buffer".  Avoid
using GW-BASIC names like "err1:" and "err2:".  Also don't name them after the
goto location like "err_kmalloc_failed:"

The rationale for using gotos is:

- unconditional statements are easier to understand and follow
- nesting is reduced
- errors by not updating individual exit points when making
    modifications are prevented
- saves the compiler work to optimize redundant code away ;)

int fun(int a)
{
 int result = 0;
 char *buffer;

 buffer = kmalloc(SIZE, GFP_KERNEL);
 if (!buffer)
  return -ENOMEM;

 if (condition1) {
  while (loop1) {
   ...
  }
  result = 1;
  goto out_buffer;
 }
 ...
out_buffer:
 kfree(buffer);
 return result;
}

A common type of bug to be aware of it "one err bugs" which look like this:

err:
 kfree(foo->bar);
 kfree(foo);
 return ret;

The bug in this code is that on some exit paths "foo" is NULL.  Normally the
fix for this is to split it up into two error labels "err_bar:" and "err_foo:".


  Chapter 8: Commenting

Comments are good, but there is also a danger of over-commenting.  NEVER
try to explain HOW your code works in a comment: it's much better to
write the code so that the _working_ is obvious, and it's a waste of
time to explain badly written code.

Generally, you want your comments to tell WHAT your code does, not HOW.
Also, try to avoid putting comments inside a function body: if the
function is so complex that you need to separately comment parts of it,
you should probably go back to chapter 6 for a while.  You can make
small comments to note or warn about something particularly clever (or
ugly), but try to avoid excess.  Instead, put the comments at the head
of the function, telling people what it does, and possibly WHY it does
it.

When commenting the kernel API functions, please use the kernel-doc format.
See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc
for details.

Linux style for comments is the C89 "/* ... */" style.
Don't use C99-style "// ..." comments.

The preferred style for long (multi-line) comments is:

 /*
  * This is the preferred style for multi-line
  * comments in the Linux kernel source code.
  * Please use it consistently.
  *
  * Description:  A column of asterisks on the left side,
  * with beginning and ending almost-blank lines.
  */

For files in net/ and drivers/net/ the preferred style for long (multi-line)
comments is a little different.

 /* The preferred comment style for files in net/ and drivers/net
  * looks like this.
  *
  * It is nearly the same as the generally preferred comment style,
  * but there is no initial almost-blank line.
  */

It's also important to comment data, whether they are basic types or derived
types.  To this end, use just one data declaration per line (no commas for
multiple data declarations).  This leaves you room for a small comment on each
item, explaining its use.


  Chapter 9: You've made a mess of it

That's OK, we all do.  You've probably been told by your long-time Unix
user helper that "GNU emacs" automatically formats the C sources for
you, and you've noticed that yes, it does do that, but the defaults it
uses are less than desirable (in fact, they are worse than random
typing - an infinite number of monkeys typing into GNU emacs would never
make a good program).

So, you can either get rid of GNU emacs, or change it to use saner
values.  To do the latter, you can stick the following in your .emacs file:

(defun c-lineup-arglist-tabs-only (ignored)
  "Line up argument lists by tabs, not spaces"
  (let* ((anchor (c-langelem-pos c-syntactic-element))
  (column (c-langelem-2nd-pos c-syntactic-element))
  (offset (- (1+ column) anchor))
  (steps (floor offset c-basic-offset)))
    (* (max steps 1)
       c-basic-offset)))

(add-hook 'c-mode-common-hook
          (lambda ()
            ;; Add kernel style
            (c-add-style
             "linux-tabs-only"
             '("linux" (c-offsets-alist
                        (arglist-cont-nonempty
                         c-lineup-gcc-asm-reg
                         c-lineup-arglist-tabs-only))))))

(add-hook 'c-mode-hook
          (lambda ()
            (let ((filename (buffer-file-name)))
              ;; Enable kernel mode for the appropriate files
              (when (and filename
                         (string-match (expand-file-name "~/src/linux-trees")
                                       filename))
                (setq indent-tabs-mode t)
                (setq show-trailing-whitespace t)
                (c-set-style "linux-tabs-only")))))

This will make emacs go better with the kernel coding style for C
files below ~/src/linux-trees.

But even if you fail in getting emacs to do sane formatting, not
everything is lost: use "indent".

Now, again, GNU indent has the same brain-dead settings that GNU emacs
has, which is why you need to give it a few command line options.
However, that's not too bad, because even the makers of GNU indent
recognize the authority of K&R (the GNU people aren't evil, they are
just severely misguided in this matter), so you just give indent the
options "-kr -i8" (stands for "K&R, 8 character indents"), or use
"scripts/Lindent", which indents in the latest style.

"indent" has a lot of options, and especially when it comes to comment
re-formatting you may want to take a look at the man page.  But
remember: "indent" is not a fix for bad programming.


  Chapter 10: Kconfig configuration files

For all of the Kconfig* configuration files throughout the source tree,
the indentation is somewhat different.  Lines under a "config" definition
are indented with one tab, while help text is indented an additional two
spaces.  Example:

config AUDIT
 bool "Auditing support"
 depends on NET
 help
   Enable auditing infrastructure that can be used with another
   kernel subsystem, such as SELinux (which requires this for
   logging of avc messages output).  Does not do system-call
   auditing without CONFIG_AUDITSYSCALL.

Seriously dangerous features (such as write support for certain
filesystems) should advertise this prominently in their prompt string:

config ADFS_FS_RW
 bool "ADFS write support (DANGEROUS)"
 depends on ADFS_FS
 ...

For full documentation on the configuration files, see the file
Documentation/kbuild/kconfig-language.txt.


  Chapter 11: Data structures

Data structures that have visibility outside the single-threaded
environment they are created and destroyed in should always have
reference counts.  In the kernel, garbage collection doesn't exist (and
outside the kernel garbage collection is slow and inefficient), which
means that you absolutely _have_ to reference count all your uses.

Reference counting means that you can avoid locking, and allows multiple
users to have access to the data structure in parallel - and not having
to worry about the structure suddenly going away from under them just
because they slept or did something else for a while.

Note that locking is _not_ a replacement for reference counting.
Locking is used to keep data structures coherent, while reference
counting is a memory management technique.  Usually both are needed, and
they are not to be confused with each other.

Many data structures can indeed have two levels of reference counting,
when there are users of different "classes".  The subclass count counts
the number of subclass users, and decrements the global count just once
when the subclass count goes to zero.

Examples of this kind of "multi-level-reference-counting" can be found in
memory management ("struct mm_struct": mm_users and mm_count), and in
filesystem code ("struct super_block": s_count and s_active).

Remember: if another thread can find your data structure, and you don't
have a reference count on it, you almost certainly have a bug.


  Chapter 12: Macros, Enums and RTL

Names of macros defining constants and labels in enums are capitalized.

#define CONSTANT 0x12345

Enums are preferred when defining several related constants.

CAPITALIZED macro names are appreciated but macros resembling functions
may be named in lower case.

Generally, inline functions are preferable to macros resembling functions.

Macros with multiple statements should be enclosed in a do - while block:

#define macrofun(a, b, c)    \
 do {     \
  if (a == 5)   \
   do_this(b, c);  \
 } while (0)

Things to avoid when using macros:

1) macros that affect control flow:

#define FOO(x)     \
 do {     \
  if (blah(x) < 0)  \
   return -EBUGGERED; \
 } while(0)

is a _very_ bad idea.  It looks like a function call but exits the "calling"
function; don't break the internal parsers of those who will read the code.

2) macros that depend on having a local variable with a magic name:

#define FOO(val) bar(index, val)

might look like a good thing, but it's confusing as hell when one reads the
code and it's prone to breakage from seemingly innocent changes.

3) macros with arguments that are used as l-values: FOO(x) = y; will
bite you if somebody e.g. turns FOO into an inline function.

4) forgetting about precedence: macros defining constants using expressions
must enclose the expression in parentheses. Beware of similar issues with
macros using parameters.

#define CONSTANT 0x4000
#define CONSTEXP (CONSTANT | 3)

The cpp manual deals with macros exhaustively. The gcc internals manual also
covers RTL which is used frequently with assembly language in the kernel.


  Chapter 13: Printing kernel messages

Kernel developers like to be seen as literate. Do mind the spelling
of kernel messages to make a good impression. Do not use crippled
words like "dont"; use "do not" or "don't" instead.  Make the messages
concise, clear, and unambiguous.

Kernel messages do not have to be terminated with a period.

Printing numbers in parentheses (%d) adds no value and should be avoided.

There are a number of driver model diagnostic macros in <linux/device.h>
which you should use to make sure messages are matched to the right device
and driver, and are tagged with the right level:  dev_err(), dev_warn(),
dev_info(), and so forth.  For messages that aren't associated with a
particular device, <linux/printk.h> defines pr_notice(), pr_info(),
pr_warn(), pr_err(), etc.

Coming up with good debugging messages can be quite a challenge; and once
you have them, they can be a huge help for remote troubleshooting.  However
debug message printing is handled differently than printing other non-debug
messages.  While the other pr_XXX() functions print unconditionally,
pr_debug() does not; it is compiled out by default, unless either DEBUG is
defined or CONFIG_DYNAMIC_DEBUG is set.  That is true for dev_dbg() also,
and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
the ones already enabled by DEBUG.

Many subsystems have Kconfig debug options to turn on -DDEBUG in the
corresponding Makefile; in other cases specific files #define DEBUG.  And
when a debug message should be unconditionally printed, such as if it is
already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
used.


  Chapter 14: Allocating memory

The kernel provides the following general purpose memory allocators:
kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
vzalloc().  Please refer to the API documentation for further information
about them.

The preferred form for passing a size of a struct is the following:

 p = kmalloc(sizeof(*p), ...);

The alternative form where struct name is spelled out hurts readability and
introduces an opportunity for a bug when the pointer variable type is changed
but the corresponding sizeof that is passed to a memory allocator is not.

Casting the return value which is a void pointer is redundant. The conversion
from void pointer to any other pointer type is guaranteed by the C programming
language.

The preferred form for allocating an array is the following:

 p = kmalloc_array(n, sizeof(...), ...);

The preferred form for allocating a zeroed array is the following:

 p = kcalloc(n, sizeof(...), ...);

Both forms check for overflow on the allocation size n * sizeof(...),
and return NULL if that occurred.


  Chapter 15: The inline disease

There appears to be a common misperception that gcc has a magic "make me
faster" speedup option called "inline". While the use of inlines can be
appropriate (for example as a means of replacing macros, see Chapter 12), it
very often is not. Abundant use of the inline keyword leads to a much bigger
kernel, which in turn slows the system as a whole down, due to a bigger
icache footprint for the CPU and simply because there is less memory
available for the pagecache. Just think about it; a pagecache miss causes a
disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
that can go into these 5 milliseconds.

A reasonable rule of thumb is to not put inline at functions that have more
than 3 lines of code in them. An exception to this rule are the cases where
a parameter is known to be a compiletime constant, and as a result of this
constantness you *know* the compiler will be able to optimize most of your
function away at compile time. For a good example of this later case, see
the kmalloc() inline function.

Often people argue that adding inline to functions that are static and used
only once is always a win since there is no space tradeoff. While this is
technically correct, gcc is capable of inlining these automatically without
help, and the maintenance issue of removing the inline when a second user
appears outweighs the potential value of the hint that tells gcc to do
something it would have done anyway.


  Chapter 16: Function return values and names

Functions can return values of many different kinds, and one of the
most common is a value indicating whether the function succeeded or
failed.  Such a value can be represented as an error-code integer
(-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure,
non-zero = success).

Mixing up these two sorts of representations is a fertile source of
difficult-to-find bugs.  If the C language included a strong distinction
between integers and booleans then the compiler would find these mistakes
for us... but it doesn't.  To help prevent such bugs, always follow this
convention:

 If the name of a function is an action or an imperative command,
 the function should return an error-code integer.  If the name
 is a predicate, the function should return a "succeeded" boolean.

For example, "add work" is a command, and the add_work() function returns 0
for success or -EBUSY for failure.  In the same way, "PCI device present" is
a predicate, and the pci_dev_present() function returns 1 if it succeeds in
finding a matching device or 0 if it doesn't.

All EXPORTed functions must respect this convention, and so should all
public functions.  Private (static) functions need not, but it is
recommended that they do.

Functions whose return value is the actual result of a computation, rather
than an indication of whether the computation succeeded, are not subject to
this rule.  Generally they indicate failure by returning some out-of-range
result.  Typical examples would be functions that return pointers; they use
NULL or the ERR_PTR mechanism to report failure.


  Chapter 17:  Don't re-invent the kernel macros

The header file include/linux/kernel.h contains a number of macros that
you should use, rather than explicitly coding some variant of them yourself.
For example, if you need to calculate the length of an array, take advantage
of the macro

  #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))

Similarly, if you need to calculate the size of some structure member, use

  #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))

There are also min() and max() macros that do strict type checking if you
need them.  Feel free to peruse that header file to see what else is already
defined that you shouldn't reproduce in your code.


  Chapter 18:  Editor modelines and other cruft

Some editors can interpret configuration information embedded in source files,
indicated with special markers.  For example, emacs interprets lines marked
like this:

-*- mode: c -*-

Or like this:

/*
Local Variables:
compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
End:
*/

Vim interprets markers that look like this:

/* vim:set sw=8 noet */

Do not include any of these in source files.  People have their own personal
editor configurations, and your source files should not override them.  This
includes markers for indentation and mode configuration.  People may use their
own custom mode, or may have some other magic method for making indentation
work correctly.


  Chapter 19:  Inline assembly

In architecture-specific code, you may need to use inline assembly to interface
with CPU or platform functionality.  Don't hesitate to do so when necessary.
However, don't use inline assembly gratuitously when C can do the job.  You can
and should poke hardware from C when possible.

Consider writing simple helper functions that wrap common bits of inline
assembly, rather than repeatedly writing them with slight variations.  Remember
that inline assembly can use C parameters.

Large, non-trivial assembly functions should go in .S files, with corresponding
C prototypes defined in C header files.  The C prototypes for assembly
functions should use "asmlinkage".

You may need to mark your asm statement as volatile, to prevent GCC from
removing it if GCC doesn't notice any side effects.  You don't always need to
do so, though, and doing so unnecessarily can limit optimization.

When writing a single inline assembly statement containing multiple
instructions, put each instruction on a separate line in a separate quoted
string, and end each string except the last with \n\t to properly indent the
next instruction in the assembly output:

 asm ("magic %reg1, #42\n\t"
      "more_magic %reg2, %reg3"
      : /* outputs */ : /* inputs */ : /* clobbers */);


  Chapter 20: Conditional Compilation

Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
files; doing so makes code harder to read and logic harder to follow.  Instead,
use such conditionals in a header file defining functions for use in those .c
files, providing no-op stub versions in the #else case, and then call those
functions unconditionally from .c files.  The compiler will avoid generating
any code for the stub calls, producing identical results, but the logic will
remain easy to follow.

Prefer to compile out entire functions, rather than portions of functions or
portions of expressions.  Rather than putting an ifdef in an expression, factor
out part or all of the expression into a separate helper function and apply the
conditional to that function.

If you have a function or variable which may potentially go unused in a
particular configuration, and the compiler would warn about its definition
going unused, mark the definition as __maybe_unused rather than wrapping it in
a preprocessor conditional.  (However, if a function or variable *always* goes
unused, delete it.)

Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
symbol into a C boolean expression, and use it in a normal C conditional:

 if (IS_ENABLED(CONFIG_SOMETHING)) {
  ...
 }

The compiler will constant-fold the conditional away, and include or exclude
the block of code just as with an #ifdef, so this will not add any runtime
overhead.  However, this approach still allows the C compiler to see the code
inside the block, and check it for correctness (syntax, types, symbol
references, etc).  Thus, you still have to use an #ifdef if the code inside the
block references symbols that will not exist if the condition is not met.

At the end of any non-trivial #if or #ifdef block (more than a few lines),
place a comment after the #endif on the same line, noting the conditional
expression used.  For instance:

#ifdef CONFIG_SOMETHING
...
#endif /* CONFIG_SOMETHING */


  Appendix I: References

The C Programming Language, Second Edition
by Brian W. Kernighan and Dennis M. Ritchie.
Prentice Hall, Inc., 1988.
ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
URL: http://cm.bell-labs.com/cm/cs/cbook/

The Practice of Programming
by Brian W. Kernighan and Rob Pike.
Addison-Wesley, Inc., 1999.
ISBN 0-201-61586-X.
URL: http://cm.bell-labs.com/cm/cs/tpop/

GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
gcc internals and indent, all available from http://www.gnu.org/manual/

WG14 is the international standardization working group for the programming
language C, URL: http://www.open-std.org/JTC1/SC22/WG14/

Kernel CodingStyle, by greg@kroah.com at OLS 2002:
http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/ 

Reference: 

Linux Kernel Coding Style