PEP: 8
Title: Style Guide for Python Code
Version: $Revision$
Last-Modified: $Date$
Author: Guido van Rossum <guido@python.org>,
        Barry Warsaw <barry@python.org>,
        Nick Coghlan <ncoghlan@gmail.com>
Status: Active
Type: Process
Content-Type: text/x-rst
Created: 05-Jul-2001
Post-History: 05-Jul-2001, 01-Aug-2013


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 the most important.

However, know when to be inconsistent -- sometimes style guide
recommendations just aren't applicable.  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* [#fn-hi]_.  When using a hanging
indent the following should be considered; 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)

.. _`multiline if-statements`:

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()

(Also see the discussion of whether to break before or after binary
operators below.)

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.


Should a line break before or after a binary operator?
------------------------------------------------------

For decades the recommended style was to break after binary operators.
But this can hurt readability in two ways: the operators tend to get
scattered across different columns on the screen, and each operator is
moved away from its operand and onto the previous line.  Here, the eye
has to do extra work to tell which items are added and which are
subtracted::

    # No: operators sit far away from their operands
    income = (gross_wages +
              taxable_interest +
              (dividends - qualified_dividends) -
              ira_deduction -
              student_loan_interest)

To solve this readability problem, mathematicians and their publishers
follow the opposite convention.  Donald Knuth explains the traditional
rule in his *Computers and Typesetting* series: "Although formulas
within a paragraph always break after binary operations and relations,
displayed formulas always break before binary operations" [3]_.

Following the tradition from mathematics usually results in more
readable code::

    # Yes: easy to match operators with operands
    income = (gross_wages
              + taxable_interest
              + (dividends - qualified_dividends)
              - ira_deduction
              - student_loan_interest)

In Python code, it is permissible to break before or after a binary
operator, as long as the convention is consistent locally.  For new
code Knuth's style is suggested.


Blank Lines
-----------

Surround top-level function and class definitions with two blank
lines.

Method definitions inside a class are surrounded 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.

- 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.


Module level dunder names
-------------------------

Module level "dunders" (i.e. names with two leading and two trailing
underscores) such as ``__all__``, ``__author__``, ``__version__``,
etc. should be placed after the module docstring but before any import
statements *except* ``from __future__`` imports.  Python mandates that
future-imports must appear in the module before any other code except
docstrings.

For example::

    """This is the example module.

    This module does stuff.
    """

    from __future__ import barry_as_FLUFL

    __all__ = ['a', 'b', 'c']
    __version__ = '0.1'
    __author__ = 'Cardinal Biggles'

    import os
    import sys


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
---------------------

- Avoid trailing whitespace anywhere.  Because it's usually invisible,
  it can be confusing: e.g. a backslash followed by a space and a
  newline does not count as a line continuation marker.  Some editors
  don't preserve it and many projects (like CPython itself) have
  pre-commit hooks that reject it.

- 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)

- Function annotations should use the normal rules for colons and
  always have spaces around the ``->`` arrow if present.  (See
  `Function Annotations`_ below for more about function annotations.)

  Yes::

      def munge(input: AnyStr): ...
      def munge() -> AnyStr: ...

  No::

      def munge(input:AnyStr): ...
      def munge()->PosInt: ...

- When combining an argument annotation with a default value, use
  spaces around the ``=`` sign (but only for those arguments that have
  both an annotation and a default).

  Yes::

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

  No::

      def munge(input: AnyStr=None): ...
      def munge(input: AnyStr, limit = 1000): ...

- 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()

Comments
========

Comments that contradict the code are worse than no comments.  Always
make a priority of keeping the comments up-to-date when the code
changes!

Comments should be complete sentences.  If a comment is a phrase or
sentence, its first word should be capitalized, unless it is an
identifier that begins with a lower case letter (never alter the case
of identifiers!).

If a comment is short, the period at the end can be omitted.  Block
comments generally consist of one or more paragraphs built out of
complete sentences, and each sentence should end in a period.

You should use two spaces after a sentence-ending period.

When writing English, follow Strunk and White.

Python coders from non-English speaking countries: please write your
comments in English, unless you are 120% sure that the code will never
be read by people who don't speak your language.

Block Comments
--------------

Block comments generally apply to some (or all) code that follows
them, and are indented to the same level as that code.  Each line of a
block comment starts with a ``#`` and a single space (unless it is
indented text inside the comment).

Paragraphs inside a block comment are separated by a line containing a
single ``#``.

Inline Comments
---------------

Use inline comments sparingly.

An inline comment is a comment on the same line as a statement.
Inline comments should be separated by at least two spaces from the
statement.  They should start with a # and a single space.

Inline comments are unnecessary and in fact distracting if they state
the obvious.  Don't do this::

    x = x + 1                 # Increment x

But sometimes, this is useful::

    x = x + 1                 # Compensate for border

Documentation Strings
---------------------

Conventions for writing good documentation strings
(a.k.a. "docstrings") are immortalized in PEP 257.

- Write docstrings for all public modules, functions, classes, and
  methods.  Docstrings are not necessary for non-public methods, but
  you should have a comment that describes what the method does.  This
  comment should appear after the ``def`` line.

- PEP 257 describes good docstring conventions.  Note that most
  importantly, the ``"""`` that ends a multiline docstring should be
  on a line by itself, e.g.::

      """Return a foobang

      Optional plotz says to frobnicate the bizbaz first.
      """

- For one liner docstrings, please keep the closing ``"""`` on
  the same line.


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 [4]_).  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.

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.

- Be consistent in return statements.  Either all return statements in
  a function should return an expression, or none of them should.  If
  any return statement returns an expression, any return statements
  where no value is returned should explicitly state this as ``return
  None``, and an explicit return statement should be present at the
  end of the function (if reachable).

  Yes::

      def foo(x):
          if x >= 0:
              return math.sqrt(x)
          else:
              return None

      def bar(x):
          if x < 0:
              return None
          return math.sqrt(x)

  No::

      def foo(x):
          if x >= 0:
              return math.sqrt(x)

      def bar(x):
          if x < 0:
              return
          return math.sqrt(x)

- 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:

Function Annotations
--------------------

With the acceptance of PEP 484, the style rules for function
annotations are changing.

- In order to be forward compatible, function annotations in Python 3
  code should preferably use PEP 484 syntax.  (There are some
  formatting recommendations for annotations in the previous section.)

- The experimentation with annotation styles that was recommended
  previously in this PEP is no longer encouraged.

- However, outside the stdlib, experiments within the rules of PEP 484
  are now encouraged.  For example, marking up a large third party
  library or application with PEP 484 style type annotations,
  reviewing how easy it was to add those annotations, and observing
  whether their presence increases code understandabilty.

- The Python standard library should be conservative in adopting such
  annotations, but their use is allowed for new code and for big
  refactorings.

- For code that wants to make a different use of function annotations
  it is recommended to put a comment of the form::

    # type: ignore

  near the top of the file; this tells type checker to ignore all
  annotations.  (More fine-grained ways of disabling complaints from
  type checkers can be found in PEP 484.)

- Like linters, type checkers are optional, separate tools.  Python
  interpreters by default should not issue any messages due to type
  checking and should not alter their behavior based on annotations.

- Users who don't want to use type checkers are free to ignore them.
  However, it is expected that users of third party library packages
  may want to run type checkers over those packages.  For this purpose
  PEP 484 recommends the use of stub files: .pyi files that are read
  by the type checker in preference of the corresponding .py files.
  Stub files can be distributed with a library, or separately (with
  the library author's permission) through the typeshed repo [5]_.

- For code that needs to be backwards compatible, function annotations
  can be added in the form of comments.  See the relevant section of
  PEP 484 [6]_.


.. rubric:: Footnotes

.. [#fn-hi] *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] Donald Knuth's *The TeXBook*, pages 195 and 196.

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

.. [5] Typeshed repo
   https://github.com/python/typeshed

.. [6] Suggested syntax for Python 2.7 and straddling code
   https://www.python.org/dev/peps/pep-0484/#suggested-syntax-for-python-2-7-and-straddling-code



Copyright
=========

This document has been placed in the public domain.



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