RUF027 false negative on method calls

[ocaballeror]

From what I read in the original PR, the rule is meant to ignore string literals that have an immediate method call on them to avoid false positives, but from what I'm seeing, every line with a method call is completely ignored, even if the method is not invoked on the string itself. Is this intended behavior?

Sample code:

import logging


class Test:
    def method(self, arg):
        pass


name = "john"

print("hello {name}")  # correctly fails
logging.info("hello {name}")  # should fail but doesn't
Test().method("hello {name}")  # also should fail but doesn't

Invoked with:

ruff --isolated --select RUF027 --preview thing.py

Ruff version:

ruff 0.2.1

[MichaReiser]

It seems intentional. This is from the documentation explaining when the rule doesn't trigger.

/// 3. The literal (or a parent expression of the literal) has a direct method call on it (for example: "{value}".format(...))

@snowsignal can you help us understand the motivation behind it?

[ocaballeror]

What does a "parent expresssion of the literal" mean exactly? If it means it won't trigger on things like logging.info("{var}"), then the value of this rule decreases significantly IMO.

Shouldn't it only count if a method is called on the string itself?

[snowsignal]

Shouldn't it only count if a method is called on the string itself?

You're right, this rule only applies to the string itself, plus any parent expressions that the string is in. The example you gave is definitely something we should catch. Luckily, I think I have a good idea of why this is happening, and I'll try to make a fix for this soon.

A parent expression of the literal would be something like func("{name}") - something like logging.info("{var}") shouldn't be excluded by this rule, because we aren't calling a method on info("{var}") - rather, it is the method that is being called.