Error / Violation Codes¶

Flake8 and its plugins assign a code to each message that we refer to as a error code (or violation). Most plugins will list their error codes in their documentation or README.

Flake8 installs pycodestyle, pyflakes, and mccabe by default and generates its own error codes for pyflakes:

Code	Example Message
F401	`module` imported but unused
F402	import `module` from line `N` shadowed by loop variable
F403	‘from `module` import *’ used; unable to detect undefined names
F404	future import(s) `name` after other statements
F405	`name` may be undefined, or defined from star imports: `module`
F406	‘from `module` import *’ only allowed at module level
F407	an undefined `__future__` feature name was imported

F601	dictionary key `name` repeated with different values
F602	dictionary key variable `name` repeated with different values
F621	too many expressions in an assignment with star-unpacking
F622	two or more starred expressions in an assignment `(a, b, c = d)`
F631	assertion test is a tuple, which are always `True`
F632	use `==/!=` to compare `str`, `bytes`, and `int` literals
F633	assertion test is a tuple, which are always `True`

F701	a `break` statement outside of a `while` or `for` loop
F702	a `continue` statement outside of a `while` or `for` loop
F703	a `continue` statement in a `finally` block in a loop
F704	a `yield` or `yield from` statement outside of a function
F705	a `return` statement with arguments inside a generator
F706	a `return` statement outside of a function/method
F707	an `except:` block as not the last exception handler
F721	syntax error in doctest
F722	syntax error in forward annotation
F723	syntax error in type comment

F811	redefinition of unused `name` from line `N`
F812	list comprehension redefines `name` from line `N`
F821	undefined name `name`
F822	undefined name `name` in `__all__`
F823	local variable `name` … referenced before assignment
F831	duplicate argument `name` in function definition
F841	local variable `name` is assigned to but never used

F901	`raise NotImplemented` should be `raise NotImplementedError`

Note that some of these entries behave differently on Python 2 and Python 3, for example F812 is specific to Python 2 only.

We also report one extra error: E999. We report E999 when we fail to compile a file into an Abstract Syntax Tree for the plugins that require it.

mccabe only ever reports one violation - C901 based on the complexity value provided by the user.

Users should also reference pycodestyle’s list of error codes.