source-academy / py-slang / 32939238869

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Stepper: a def-local reassignment shadows the whole function body (#448)

* Stepper: a def-local reassignment shadows the whole function body (#447)

Python decides a name is local to a function from every assignment in its
body, not just the ones textually before a given read. The substitution
stepper's substitute() only skipped a def's own name/parameters when
deciding whether to substitute an outer binding into its body, so an outer
z = 2 was baked into print(z) even though f() later reassigns z, making z
local throughout f and that read an UnboundLocalError in real Python.

substitute() now also skips a def whose body assigns the name anywhere
(ast.ts's new assignedNamesOf, a recursive scan reusing declaredNamesOf).
The now-unsubstituted identifier is caught at reduction time: reduceExpr's
Identifier case raises UnboundLocalError for any non-builtin bare name
instead of treating it as an inert value, and statement-level assignment
binding (isReadyValue) forces that same check before aliasing such a name
into another variable, so the error can't be silently bypassed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* Stepper: fix formatting, self-name shadowing, and NameError vs UnboundLocalError

Addresses CodeRabbit review on #448:

- prettier formatting (a wrapped line in the new getSteps.test.ts describe
  block failed yarn format:ci).
- A def that reassigns its own name (def f(): print(f); f = 3) was still
  resolving the early f to the function value, since contractCall's selfName
  binding substituted unconditionally rather than checking assignedNamesOf
  like substitute()'s own FunctionDeclaration case does.
- Every unresolved non-builtin identifier was reported as UnboundLocalError,
  even a name never bound anywhere in the program (a NameError in real
  Python). substitute/markUnboundLocal (ast.ts) now tag specifically the
  occurrences left behind for local-shadowing reasons with an `unboundLocal`
  flag; reduceExpr's Identifier... (continued)

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