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 :pep:`style guidelines for the C code
in the C implementation of Python <7>`.
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::
# Correct:
# Aligned with opening delimiter.
foo = long_function_name(var_one, var_two,
var_three, var_four)
# Add 4 spaces (an extra level of indentation) to distinguish arguments 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)
::
# Wrong:
# 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 multiline 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 multiline 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 disallows mixing tabs and spaces for indentation.
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 line length limit up 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 could not use implicit continuation
before Python 3.10, so backslashes were acceptable for that case::
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::
# Wrong:
# 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::
# Correct:
# 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, and should not
have an encoding declaration.
In the standard library, non-UTF-8 encodings should be used only for
test purposes. Use non-ASCII characters sparingly, preferably only to
denote places and human names. If using non-ASCII characters as data,
avoid noisy Unicode characters like z̯̯͡a̧͎̺l̡͓̫g̹̲o̡̼̘ and byte order
marks.
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).
Open source projects with a global audience are encouraged to adopt a
similar policy.
Imports
-------
- Imports should usually be on separate lines::
# Correct:
import os
import sys
::
# Wrong:
import sys, os
It's okay to say this though::
# Correct:
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.
- 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 explicitly::
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::
"""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::
# Correct:
spam(ham[1], {eggs: 2})
::
# Wrong:
spam( ham[ 1 ], { eggs: 2 } )
- Between a trailing comma and a following close parenthesis::
# Correct:
foo = (0,)
::
# Wrong:
bar = (0, )
- Immediately before a comma, semicolon, or colon::
# Correct:
if x == 4: print(x, y); x, y = y, x
::
# Wrong:
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::
# Correct:
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]
::
# Wrong:
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::
# Correct:
spam(1)
::
# Wrong:
spam (1)
- Immediately before the open parenthesis that starts an indexing or
slicing::
# Correct:
dct['key'] = lst[index]
::
# Wrong:
dct ['key'] = lst [index]
- More than one space around an assignment (or other) operator to
align it with another::
# Correct:
x = 1
y = 2
long_variable = 3
::
# Wrong:
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::
# Correct:
i = i + 1
submitted += 1
x = x*2 - 1
hypot2 = x*x + y*y
c = (a+b) * (a-b)
::
# Wrong:
i=i+1
submitted +=1
x = x * 2 - 1
hypot2 = x * x + y * y
c = (a + b) * (a - b)
- 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.)::
# Correct:
def munge(input: AnyStr): ...
def munge() -> PosInt: ...
::
# Wrong:
def munge(input:AnyStr): ...
def munge()->PosInt: ...
- Don't use spaces around the ``=`` sign when used to indicate a
keyword argument, or when used to indicate a default value for an
*unannotated* function parameter::
# Correct:
def complex(real, imag=0.0):
return magic(r=real, i=imag)
::
# Wrong:
def complex(real, imag = 0.0):
return magic(r = real, i = imag)
When combining an argument annotation with a default value, however, do use
spaces around the ``=`` sign::
# Correct:
def munge(sep: AnyStr = None): ...
def munge(input: AnyStr, sep: AnyStr = None, limit=1000): ...
::
# Wrong:
def munge(input: AnyStr=None): ...
def munge(input: AnyStr, limit = 1000): ...
- Compound statements (multiple statements on the same line) are
generally discouraged::
# Correct:
if foo == 'blah':
do_blah_thing()
do_one()
do_two()
do_three()
Rather not::
# Wrong:
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::
# Wrong:
if foo == 'blah': do_blah_thing()
for x in lst: total += x
while t < 10: t = delay()
Definitely not::
# Wrong:
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()
When to Use Trailing Commas
===========================
Trailing commas are usually optional, except they are mandatory when
making a tuple of one element. For clarity, it is recommended to
surround the latter in (technically redundant) parentheses::
# Correct:
FILES = ('setup.cfg',)
::
# Wrong:
FILES = 'setup.cfg',
When trailing commas are redundant, they are often helpful when a
version control system is used, when a list of values, arguments or
imported items is expected to be extended over time. The pattern is
to put each value (etc.) on a line by itself, always adding a trailing
comma, and add the close parenthesis/bracket/brace on the next line.
However it does not make sense to have a trailing comma on the same
line as the closing delimiter (except in the above case of singleton
tuples)::
# Correct:
FILES = [
'setup.cfg',
'tox.ini',
]
initialize(FILES,
error=True,
)
::
# Wrong:
FILES = ['setup.cfg', 'tox.ini',]
initialize(FILES, error=True,)
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. The first word should be
capitalized, unless it is an identifier that begins with a lower case
letter (never alter the case of identifiers!).
Block comments generally consist of one or more paragraphs built out of
complete sentences, with each sentence ending in a period.
You should use two spaces after a sentence-ending period in multi-
sentence comments, except after the final sentence.
Ensure that your comments are clear and easily understandable to other
speakers of the language you are writing in.
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::
"""Return a foobang
Optional plotz says to frobnicate the bizbaz first.
"""
- For one liner docstrings, please keep the closing ``"""`` on
the same line::
"""Return an ex-parrot."""
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 acronyms in CapWords, capitalize all the
letters of the acronym. 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 names start
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.
ASCII Compatibility
~~~~~~~~~~~~~~~~~~~
Identifiers used in the standard library must be ASCII compatible
as described in the
:pep:`policy section <3131#policy-specification>`
of :pep:`3131`.
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.
Type Variable Names
~~~~~~~~~~~~~~~~~~~
Names of type variables introduced in :pep:`484` should normally use CapWords
preferring short names: ``T``, ``AnyStr``, ``Num``. It is recommended to add
suffixes ``_co`` or ``_contra`` to the variables used to declare covariant
or contravariant behavior correspondingly::
from typing import TypeVar
VT_co = TypeVar('VT_co', covariant=True)
KT_contra = TypeVar('KT_contra', contravariant=True)
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 and Variable Names
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Function names should be lowercase, with words separated by
underscores as necessary to improve readability.
Variable names follow the same convention as function names.
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 backwards 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: Try to keep the functional behavior side-effect free,
although side-effects such as caching are generally fine.
Note 2: 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::
# Correct:
if foo is not None:
::
# Wrong:
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::
# Correct:
def f(x): return 2*x
::
# Wrong:
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. ``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 from
None``), 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 catching exceptions, mention specific exceptions whenever
possible instead of using a bare ``except:`` clause::
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 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::
# Correct:
try:
value = collection[key]
except KeyError:
return key_not_found(key)
else:
return handle_value(value)
::
# Wrong:
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::
# Correct:
with conn.begin_transaction():
do_stuff_in_transaction(conn)
::
# Wrong:
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)::
# Correct:
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)
::
# Wrong:
def foo(x):
if x >= 0:
return math.sqrt(x)
def bar(x):
if x < 0:
return
return math.sqrt(x)
- Use ``''.startswith()`` and ``''.endswith()`` instead of string
slicing to check for prefixes or suffixes.
startswith() and endswith() are cleaner and less error prone::
# Correct:
if foo.startswith('bar'):
::
# Wrong:
if foo[:3] == 'bar':
- Object type comparisons should always use isinstance() instead of
comparing types directly::
# Correct:
if isinstance(obj, int):
::
# Wrong:
if type(obj) is type(1):
- For sequences, (strings, lists, tuples), use the fact that empty
sequences are false::
# Correct:
if not seq:
if seq:
::
# Wrong:
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 ``==``::
# Correct:
if greeting:
::
# Wrong:
if greeting == True:
Worse::
# Wrong:
if greeting is True:
- Use of the flow control statements ``return``/``break``/``continue``
within the finally suite of a ``try...finally``, where the flow control
statement would jump outside the finally suite, is discouraged. This
is because such statements will implicitly cancel any active exception
that is propagating through the finally suite::
# Wrong:
def foo():
try:
1 / 0
finally:
return 42
Function Annotations
--------------------
With the acceptance of :pep:`484`, the style rules for function
annotations have changed.
- Function annotations should 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 understandability.
- 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 checkers 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]_.
Variable Annotations
--------------------
:pep:`526` introduced variable annotations. The style recommendations for them are
similar to those on function annotations described above:
- Annotations for module level variables, class and instance variables,
and local variables should have a single space after the colon.
- There should be no space before the colon.
- If an assignment has a right hand side, then the equality sign should have
exactly one space on both sides::
# Correct:
code: int
class Point:
coords: Tuple[int, int]
label: str = '<unknown>'
::
# Wrong:
code:int # No space after colon
code : int # Space before colon
class Test:
result: int=0 # No spaces around equality sign
- Although the :pep:`526` is accepted for Python 3.6, the variable annotation
syntax is the preferred syntax for stub files on all versions of Python
(see :pep:`484` for details).
.. 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
==========
.. [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
Copyright
=========
This document has been placed in the public domain.
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