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type narrowing predicates

basedpython spells PEP 742 TypeIs[T] as name is T in the return annotation, naming the parameter being narrowed:

def is_str(x) -> x is str:
    return isinstance(x, str)

transpiles to:

from typing_extensions import TypeIs

def is_str(x) -> TypeIs[str]:
    return isinstance(x, str)

semantics

the runtime semantics are exactly PEP 742 — the function asserts that its argument has type T when it returns True, and ty narrows accordingly at call sites. the parameter name is lost in lowering (TypeIs doesn't carry it) but is preserved in the source for readers

scope

the rewrite fires only when the return annotation is a single name is T comparison where name is a bare identifier. this disambiguates from identity checks elsewhere in the function:

  • in the return annotation: x is str → TypeIs[str]
  • anywhere else: x is y follows the identity-swap rules and lowers to isinstance(x, y)

chained comparisons (a is int is str) and non-identifier left operands are ignored