UP031 not violated for variables, only literal values

[lev-blit]

UP031 (printf-string-formatting, suggests using str.format instead of percent formatting) doesn't trigger when using a variable and not a literal value.

Even using the example from the docs -

value = 1
print("%s" % value)  # seems to be OK

print("%s" % 1)  # violating UP031

I've tried this out in the rust playground as well (here's an example) with version 0.3.0, selecting all UP rules

[charliermarsh]

I believe the docs are wrong the rule is correct. It's not safe to inject an arbitrary variable unless we can definitively infer its type. Here's an example to illustrate why:

>>> value = 1
>>> print("%s" % value)
1
>>> value = (1,)
>>> print("%s" % value)
1
>>> value = 1
>>> print(f"{value}")
1
>>> value = (1,)
>>> print(f"{value}")
(1,)

We could improve the logic a bit to allow replacements in cases in which we can definitively infer the type (like above), but in general we won't always be able to do this conversion.

[zanieb]

The documentation part seems like a good first issue. Inference of the type seems a bit harder to implement but definitely doable if someone is interested.

[omar-abdelgawad]

Hi, I am interested in taking the documentation part of this issue. I am also new here 😄.

However, I need some clarifications regarding what needs to be updated. From what I have understood, The rule implementation is correct and it is successful in detecting both safe (when we know its definitely not a tuple) and unsafe (when there is a chance it might be a tuple) fixes. The only part missing from the rule is further type detection to find all cases where the variable is NOT a tuple. The thing is, when I read the docs now I understand the exact same thing with no mistakes found.

Did some PR fix this already and forget to mention this issue or am I just confused?

[zanieb]

Hey @omar-abdelgawad — you're right this example from the docs is already a part of the "Known problems" section. I don't think there's a documentation change to be made here until we improve inference.

[zanieb]

Welcome to the project :) sorry about the confusion.