Failed to create fix for UnnecessaryMap: Currently not supporting default values

[qarmin]

Ruff 0.0.285

ruff  *.py --select ALL

file content:

_ = f"{dict(map(lambda v: (v, v**2), nums))}"
# For simple expressions, this could be: `(x if x else 1 for x in nums)`.
map(lambda x=1: x, nums)

error:

error: Failed to create fix for UnnecessaryMap: Currently not supporting default values

For me, this error should not be visible for user, because this message is useless for it

PY_FILE_TEST_178904787.py.zip

[dhruvmanila]

The fix might not work if there are any falsy values in the iterable. For example, if one of the values of nums is 0 then it would use 1 but it explicitly passed the argument.

I do agree that the message isn't useful from user perspective and instead we should avoid auto-generating the fix.

[charliermarsh]

Yeah it's fine that this is unsupported, but we should attempting to fix it and thus raising the message.

[charliermarsh]

The fix might not work if there are any falsy values in the iterable. For example, if one of the values of nums is 0 then it would use 1 but it explicitly passed the argument.

Can you expand on this with an example?

[dhruvmanila]

Here, you can see the output is different. Default values in this context will never be used as map will always pass a value to the function.

In [1]: nums = [0, 1, 2]

In [2]: list(map(lambda x=1: x, nums))
Out[2]: [0, 1, 2]

In [3]: list(x if x else 1 for x in nums)
Out[3]: [1, 1, 2]

[charliermarsh]

But, isn't this suggesting that the user change list(map(lambda x=1: x, nums)) to [x for x in nums]? Why the inclusion of if x else 1?

[dhruvmanila]

Oh, it's mentioned in the PR description:

For simple expressions, this could be: (x if x else 1 for x in nums).

But yeah, your suggestion would work correctly.

[charliermarsh]

We should probably just skip attempting the fix if there are default values. It's an unusual pattern and I can think of a few cases where it would cause problems.

[dhruvmanila]

Yeah, I'm thinking of the same solution.