Flake8 failing to detect syntax issues that can be detect by pyflakes - flake8

Question

I am trying to get Emacs' flycheck to work nicely with python syntax error checking. By default it uses flake8, but I noticed that recently flake8 is only reporting style issues, when it supposed to produce the same kind of results as pyflakes.
As an example, I have a simple python script with a wrong module name.
$ cat so.py
import os
print(os.path)
op.path()
output of $ flake8 so.py is empty, while pyflakes' output is
$ pyflakes so.py
so.py:5: undefined name 'op'
My flake8 configuration, if it matters:
[flake8]
max-line-length = 96
ignore = E123,E701,E126,F821
I am using Linux, if that matters.
Flake8 version: version 3.3.0 mccabe: 0.6.1, pycodestyle: 2.3.1, pyflakes: 1.5.0
Pyflakes version: 1.5.0

Answer 1

When you're wondering why Flake8 isn't reporting something you think it should, the best path forward is to utilize the --isolated flag.
With the file contents above in ex.py if I do:
❯❯❯ flake8 --isolated ex.py
ex.py:5:1: F821 undefined name 'op'
With that in mind, it does appear that your configuration is the problem here.