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assignment-in-assert (RUF018)

Added in v0.2.0 · Related issues · View source

What it does

Checks for named assignment expressions (e.g., x := 0) in assert statements.

Why is this bad?

Named assignment expressions (also known as "walrus operators") are used to assign a value to a variable as part of a larger expression.

Named assignments are syntactically valid in assert statements. However, when the Python interpreter is run under the -O flag, assert statements are not executed. In this case, the named assignment will also be ignored, which may result in unexpected behavior (e.g., undefined variable accesses).

Example

assert (x := 0) == 0
print(x)

Use instead:

x = 0
assert x == 0
print(x)

The rule avoids flagging named expressions that define variables which are only referenced from inside assert statements; the following will not trigger the rule:

assert (x := y**2) > 42, f"Expected >42 but got {x}"

Nor will this:

assert (x := y**2) > 42
assert x < 1_000_000

References