:= (walrus) — the assignment expression is not parsed

xs = [1, 2, 3, 4]
if (n := len(xs)) > 3:
    print("big", n)

pascal26:2: error: undefined variable (n)

The name is never bound, so the failure is a clean "undefined variable" at the USE — visible, not silent.

2 of the neuzelaar corpus's 168 files, which is why this sits low. It is listed here so the census in feature-nilpy-thirdparty-libraries-as-targets has a ticket behind every line rather than one unaccounted construct.

Notes

:= is Pascal's ASSIGNMENT token, so the token already exists — this is about NilPy's expression parser accepting a binding in expression position and registering the name in the enclosing scope (Python scopes a walrus to the enclosing function/module, NOT to the comprehension or if it appears in).

The three shapes real code uses: an if/while condition (above), a comprehension filter ([y for x in xs if (y := f(x)) is not None]), and inside a call argument.

Gate

make test-nilpy + self-host byte-identical, with a CPython-diffed test over all three shapes plus the scoping rule (the name is still bound AFTER the statement).