Rule: prefer `if` over ternary operation evaluating to `None` if cond doesn't hold

[dorschw]

Proposal

Disallow ternary operations that:

• if condition holds, call a method
• otherwise, do nothing (the whole expression evaluates toNone)

e.g. method(...) if condition else None

Why is it bad?

Can be autofixed, simplified, and be clearer for future maintainers.

Examples & autofix

- print("🐍") if condition else None
+ if condition:
+     print("🐍")

- dictionary.update({"foo":"bar"}) if condition else None
+ if condition:
+     dictionary.update({"foo":"bar"})

(the second example can be simplified to dictionary["foo"]="bar", but the example is what inspired this proposal, and the simplification doesn't serve as good example)

[wookie184]

Would a better rule just be one that just triggered if the result of a ternary is unused (i.e. on a line by itself)? I think that would still catch both of your cases, but would avoid potential false positives e.g.

maybe_int = int(s) if s.isdecimal() else None

And would also catch extra cases like:

print("hello") if cond else print("goodbye")

[wookie184]

I can have a go at implementing this, I assume it would be a RUF rule since I can't see an implementation elsewhere?

[charliermarsh]

We could include it as a RUF rule. I'd like to see if it flags anything on the ecosystem CI checks before committing to merging it. Are you open to implementing, then evaluating whether there are valid patterns that we might be overlooking based on what gets flagged?

(I think your suggestion of checking if the result is unused makes sense -- so checking for StmtExpr where the Expr is a ternary.)

[wookie184]

Yeah that sounds good, it doesn't look too difficult to implement so there'd be little loss if we decide not to continue after opening a PR 👍

[fgimian]

I think there's merit in a rule that completely disallows ternary expressions as they cannot be accurately measured by coverage.py. There's flake8-if-expr which does this.

Would it be possible to support this as part of this feature please?

Thanks heaps
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