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[ty] Use all reachable bindings for instance attributes and deferred lookups (#18955)

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## Summary

Remove a hack in control flow modeling that was treating `return`
statements at the end of function bodies in a special way (basically
considering the state *just before* the `return` statement as the
end-of-scope state). This is not needed anymore now that #18750 has been
merged.

In order to make this work, we now use *all reachable bindings* for
purposes of finding implicit instance attribute assignments as well as
for deferred lookups of symbols. Both would otherwise be affected by
this change:
```py
def C:
    def f(self):
        self.x = 1  # a reachable binding that is not visible at the end of the scope
        return
```

```py
def f():
    class X: ...  # a reachable binding that is not visible at the end of the scope
    x: "X" = X()  # deferred use of `X`
    return
```

Implicit instance attributes also required another change. We previously
kept track of possibly-unbound instance attributes in some cases, but we
now give up on that completely and always consider *implicit* instance
attributes to be bound if we see a reachable binding in a reachable
method. The previous behavior was somewhat inconsistent anyway because
we also do not consider attributes possibly-unbound in other scenarios:
we do not (and can not) keep track of whether or not methods are called
that define these attributes.

closes https://github.com/astral-sh/ty/issues/711

## Ecosystem analysis

I think this looks very positive!

* We see an unsurprising drop in `possibly-unbound-attribute`
diagnostics (599), mostly for classes that define attributes in `try …
except` blocks, `for` loops, or `if … else: raise …` constructs. There
might obviously also be true positives that got removed, but the vast
majority should be false positives.
* There is also a drop in `possibly-unresolved-reference` /
`unresolved-reference` diagnostics (279+13) from the change to deferred
lookups.
* Some `invalid-type-form` false positives got resolved (13), because we
can now properly look up the names in the annotations.
* There are some new *true* positives in `attrs`, since we understand
the `Attribute` annotation that was previously inferred as `Unknown`
because of a re-assignment after the class definition.


## Test Plan

The existing attributes.md test suite has sufficient coverage here.
This commit is contained in:
David Peter 2025-07-01 14:38:36 +02:00 • committed by GitHub
parent ebf59e2bef
commit dac4e356eb
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7 changed files with 30 additions and 70 deletions
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24
crates/ty_python_semantic/src/semantic_index/builder.rs
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@ -1122,29 +1122,7 @@ impl<'ast> Visitor<'ast> for SemanticIndexBuilder<'_, 'ast> {
&mut first_parameter_name,
);
// TODO: Fix how we determine the public types of symbols in a
// function-like scope: https://github.com/astral-sh/ruff/issues/15777
//
// In the meantime, visit the function body, but treat the last statement
// specially if it is a return. If it is, this would cause all definitions
// in the function to be marked as non-visible with our current treatment
// of terminal statements. Since we currently model the externally visible
// definitions in a function scope as the set of bindings that are visible
// at the end of the body, we then consider this function to have no
// externally visible definitions. To get around this, we take a flow
// snapshot just before processing the return statement, and use _that_ as
// the "end-of-body" state that we resolve external references against.
if let Some((last_stmt, first_stmts)) = body.split_last() {
builder.visit_body(first_stmts);
let pre_return_state = matches!(last_stmt, ast::Stmt::Return(_))
.then(|| builder.flow_snapshot());
builder.visit_stmt(last_stmt);
let reachability = builder.current_use_def_map().reachability;
if let Some(pre_return_state) = pre_return_state {
builder.flow_restore(pre_return_state);
builder.current_use_def_map_mut().reachability = reachability;
}
}
builder.visit_body(body);
builder.current_first_parameter_name = first_parameter_name;
builder.pop_scope()