Error / Violation Codes

Flake8 and its plugins assign a code to each message that we refer to as an 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

F501	invalid `%` format literal
F502	`%` format expected mapping but got sequence
F503	`%` format expected sequence but got mapping
F504	`%` format unused named arguments
F505	`%` format missing named arguments
F506	`%` format mixed positional and named arguments
F507	`%` format mismatch of placeholder and argument count
F508	`%` format with `*` specifier requires a sequence
F509	`%` format with unsupported format character
F521	`.format(...)` invalid format string
F522	`.format(...)` unused named arguments
F523	`.format(...)` unused positional arguments
F524	`.format(...)` missing argument
F525	`.format(...)` mixing automatic and manual numbering
F541	f-string without any placeholders

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 is always `True`
F632	use `==/!=` to compare `str`, `bytes`, and `int` literals
F633	use of `>>` is invalid with `print` function
F634	if test is a tuple, which is 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
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`
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`

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.