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
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
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

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.