Add warning on invalid percent markers; ValueError: unsupported format character

[asottile]

In GitLab by @pjacock on Mar 4, 2019, 06:44

Please read this brief portion of documentation before going any further: http://flake8.pycqa.org/en/latest/internal/contributing.html#filing-a-bug

Please describe how you installed Flake8

$ pip install flake8

Please describe the problem or feature

Consider this example, call it str_fmt.py:

"""String formatting test case.

The percent j should be percent i for integer.
"""

a = 123
b = "Hello"
c = "World"

print("%j\t%s\t%s\n" % (a, b, c))

Currently flake8 does not report any problem with this file:

$ flake8 str_fmt.py ; echo "Return code $?"
Return code 0

However, on running it:

$ python str_fmt.py
Traceback (most recent call last):
  File "str_fmt.py", line 10, in <module>
    print("%j\t%s\t%s\n" % (a, b, c))
ValueError: unsupported format character 'j' (0x6a) at index 1

Please provide the exact, unmodified output of flake8 --bug-report

Not a bug, but for reference:

$ flake8 --version
3.6.0 (black: 0.0.1, flake8-blind-except: 0.1.1, flake8-bugbear: 18.8.0, flake8-docstrings: 1.3.0, pydocstyle: 3.0.0, mccabe: 0.6.1, pycodestyle: 2.4.0, pyflakes: 2.0.0, rst-docstrings: 0.0.8) CPython 3.7.2 on Linux

Please describe the problem or feature

Static code analysis could spot this kind of error and prevent ValueError: unsupported format character exceptions. In my experience this kind of error is particularly common in corner cases for error handling or logging code which is not exhaustively tested.

This is analogous to the flake8 warning akin to W605 invalid escape sequence (although that does not usually trigger an exception), but for invalid percent placeholders.

[asottile]

In GitLab by @asottile on Mar 4, 2019, 06:47

Flake8 is merely a framework which other linters run under and does not implement any rules itself. I'd suggest either writing a plugin or reporting this to pyflakes / pycodestyle. I think there's something underway in pyflakes for this already : PyCQA/pyflakes#370

[asottile]

In GitLab by @asottile on Mar 4, 2019, 06:47

closed

[asottile]

In GitLab by @pjacock on Mar 4, 2019, 06:56

Thank you - I used slightly different search terms so didn't find that. Also, I wasn't sure if it would fall under pyflakes or pycodestyle, so reported it here.