Common Python Linter Warnings

Sep 11, 2023

As I mentioned previously, I'm now using Ruff to lint my Python projects. Several linter warnings continually crop up in my code, which I find interesting, so I thought I'd highlight a few of them (there are plenty that I'm leaving out; I apparently write fairly crude code by these linters' standards).

This is a common recurrence in places where I'm setting up a dict for something:

mydict = {
    'posts': some_queryset.all(),
    'today':  # Missing a trailing comma

This pops up on occasion for me, though not terribly often. I apparently easily forget about the not operator.

return False if self.errors else True

# The above is a little more legible if we use:
return not self.errors

I was surprised that this occurred so often in my code. Removing these cases flattens the code somewhat, which is a new practice I'm trying to engrain into my programming habits.

if (is_ajax_request(request)):
    return HttpResponseForbidden('Forbidden')
else:  # This isn't needed
    return redirect(reverse('home'))

# The above looks better as:
if (is_ajax_request(request)):
    return HttpResponseForbidden('Forbidden')

return redirect(reverse('home'))

This warning taught me something I didn't know about f-strings; namely that explicit conversion flags are available. Also that the conversions I was making were mostly not necessary in the first place.

error = f"Part not owned by {str(self.part_owner)}!"

# Better:
error = f"Part not owned by {self.part_owner!s}!."

# Best:
error = f"Part not owned by {self.part_owner}!."

Again, I was surprised by how often I do this. Base types can (and often are) subclassed, so it's better to use isinstance() than type is.

if (type(loader) is list):
    return error_response(loader)

# Better:
if (isinstance(loader, list)):
    return error_response(loader)

Running the linting frameworks has taught me a fair amount about my programming habits, and has also informed me about various aspects of the language. I recommend running linters if you don't already, and I highly recommend Ruff!

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