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.
These are the general code layout rules.
4 real spaces. No tabs, no exceptions.
79 characters with a soft limit for 84 if absolutely necessary. Try
to avoid deeply nested code by cleverly placing
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' ]
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'
These are some specific rules about how to deal with formatting of expressions and statements.
~etc.) as well on the inner side of parentheses.
exp = -1.05 value = (item_value / item_count) * offset / exp value = my_list[index] value = my_dict['key']
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:
if method == 'md5': pass
if 'md5' == method: pass
foo is not None)
False(for example never do
foo == False, do
Use negated containment checks: for instance
foo not in bar instead of
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.
CamelCase, with acronyms kept uppercase (
Protected members are prefixed with a single underscore. Double underscores are reserved for mixin classes.
If a name clashes with a keyword, append 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:
clsas first parameter
selfas first parameter
display_name = property(lambda x: x.real_name or x.username)
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.
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