Make flycheck mode respect flake8 config and warn for long lines

Question

I use flycheck-mode as my syntax checker/linter for Python. It works great for the most part, however I can't get it to warn me when I have gone over a specified line length.

I am using flake8 as my checker for Python, and have confirmed this with flycheck-verify-setup. The output from this command in a python-mode buffer is:

Syntax checkers for buffer config.py in python-mode:

python-flake8
- predicate:  t
- executable: Found at /usr/bin/flake8

python-pylint
- predicate:  nil
- executable: Not found

python-pycompile
- predicate:  t
- executable: Found at /usr/bin/python

The following checker is explicitly selected for this buffer:

python-flake8
- major mode: `python-mode' supported
- predicate:  t
- executable: Found at /usr/bin/flake8

Flycheck Mode is enabled.  Use C-u C-c ! x to enable disabled
checkers.

The checking works in general, so if I have the following Python code:

import re
long_string = "foooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo"
class Foo():
    pass

I get two warnings from flycheck:

're' imported but unused [F401]
expected 2 blank lines, found 0 [E302]

I do not, however, get a warning that long_string is too long. I have tried setting:

(setq-default flycheck-flake8-maximum-line-length 79)

in my init file as suggested here. I have tried configuring flake8 with a flake8 file in my ~/.config/ directory, as described here, which I assume flycheck is supposed to respect, based on this answer. I have also tried setting the value of flycheck-flake8rc to point to that flake8 file. I have tried using a local configuration file within a project as described here. None of these have worked for me. Instead, flycheck seems to ignore any flake8 config I try to give it, and will not warn about line length. This is the case even when I use emacs -Q, initialize packages, and enable global-flycheck-mode - although, in this case the output of flycheck-verify-setup is the same. Any suggestions how I might be able to:

1. Get flycheck-mode to respect my flake8 configuration file?
2. Get flycheck-mode to warn me about going over a specified line length?

Question 2 would be solved, in theory, by any solution to question 1, because I could set, e.g.:

[flake8]
max-line-length = 79

in my configuration file, but I would be satisfied with a solution to question 2 alone. Thank you!

Answer 1

I don't use flymake mode directly. But I use elpy which uses flymake internally and works well.

You can install using M-x package-install elpy.

Next time, when you open a python file, you will get a warning like this