Python Styleguide

The Pallets styleguide applies to all Pallets projects, including Flask. This styleguide is a requirement for patches to the projects and a recommendation for other code in the ecosystem.

In general the styleguide closely follows PEP 8 with some small differences and extensions.

General Layout

These are the general code layout rules.

Indentation

4 real spaces. No tabs, no exceptions.

Maximum line length

79 characters with a soft limit for 84 if absolutely necessary. Try to avoid deeply nested code by cleverly placing break, continue and return statements.

Continuing long statements

To continue a statement, use backslashes. Align the next line with the last dot or equal sign, or indent four spaces.

this_is_a_very_long(function_call, 'with many parameters') \
    .that_returns_an_object_with_an_attribute

MyModel.query.filter(MyModel.scalar > 120) \
             .order_by(MyModel.name.desc()) \
             .limit(10)

When breaking in a statement with parentheses or braces, align to the braces.

this_is_a_very_long(function_call, 'with many parameters',
                    23, 42, 'and even more')

For lists or tuples with many items, break immediately after the opening brace.

items = [
    'this is the first', 'set of items', 'with more items',
    'to come in this line', 'like this'
]

Blank lines

Top level functions and classes are separated by two lines, everything else by one. Do not use too many blank lines to separate logical segments in code.

def hello(name):
    print('Hello %s!' % name)

def goodbye(name):
    print('See you %s.' % name)
class MyClass(object):
    """This is a simple docstring"""

    def __init__(self, name):
        self.name = name

    def get_annoying_name(self):
        return self.name.upper() + '!!!!111'

Expressions and Statements

These are some specific rules about how to deal with formatting of expressions and statements.

General whitespace rules

  • No whitespace for unary operators that are not words (e.g.: -, ~ etc.) as well on the inner side of parentheses.
  • Whitespace is placed between binary operators.

Good:

exp = -1.05
value = (item_value / item_count) * offset / exp
value = my_list[index]
value = my_dict['key']

Bad:

exp = - 1.05
value = ( item_value / item_count ) * offset / exp
value = (item_value/item_count)*offset/exp
value=( item_value/item_count ) * offset/exp
value = my_list[ index ]
value = my_dict ['key']

Yoda statements are a no-go. Never compare constant with variable, always variable with constant:

Good:

if method == 'md5':
    pass

Bad:

if 'md5' == method:
    pass

Comparisons:

  • against arbitrary types: == and !=
  • against singletons with is and is not (eg: foo is not None)
  • never compare something with True or False (for example never do foo == False, do not foo instead)

Use negated containment checks: for instance foo not in bar instead of not foo in bar

For instance checks use isinstance(a, C) instead of type(A) is C, but try to avoid instance checks in general. Check for features.

Naming Conventions

  • class names: CamelCase, with acronyms kept uppercase (HTTPWriter and not HttpWriter)
  • variable names: lowercase_with_underscores
  • method and function names: lowercase_with_underscores
  • constants: UPPERCASE_WITH_UNDERSCORES
  • precompiled regular expressions: name_re

Protected members are prefixed with a single underscore. Double underscores are reserved for mixin classes.

If a name clashes with a keyword, trailing appended a trailing underscore. Clashes with builtins are allowed and must not be resolved by appending an underline to the variable name. If the function needs to access a shadowed builtin, rebind the builtin to a different name instead.

Function and method arguments:

  • class methods: cls as first parameter
  • instance methods: self as first parameter
  • lambdas for properties might have the first parameter replaced with x like in display_name = property(lambda x: x.real_name or x.username)

Docstrings

All docstrings are formatted with reStructuredText as understood by Sphinx. Depending on the number of lines in the docstring, they are laid out differently. If it's just one line, the closing triple quote is on the same line as the opening, otherwise the text is on the same line as the opening quote and the triple quote that closes the string on its own line.

def foo():
    """This is a simple docstring"""


def bar():
    """This is a longer docstring with so much information in
    there that it spans three lines.  In this case the closing
    triple quote is on its own line.
    """

The module header consists of an utf-8 encoding declaration (if non ASCII letters are used, but it is recommended all the time) and a standard docstring:

# -*- coding: utf-8 -*-
"""
    package.module
    ~~~~~~~~~~~~~~

    A brief description goes here.

    :copyright: (c) YEAR by AUTHOR.
    :license: LICENSE_NAME, see LICENSE_FILE for more details.
"""

Please keep in mind that proper copyrights and license files are a requirement for approved Flask extensions.

Comments

Rules for comments are similar to docstrings. Both are formatted with reStructuredText. If a comment is used to document an attribute, put a colon after the opening pound sign (#). If a comment is on the same line as code, use two spaces before the #.

class User(object):
    #: the name of the user as unicode string
    name = Column(String)
    #: the sha1 hash of the password + inline salt
    pw_hash = Column(String)

    _groups = None  # cache groups after loading