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iter() and iterators

  • User __iter__ and __next__ methods cannot suspend when driven by for, comprehensions, collection constructors, or native iterator adaptors. An external or OS call raises NotImplementedError inside the method, where it can be caught. Direct iter(), next() (including its default form), and functools.reduce() can suspend in these methods.

  • iter(callable, sentinel) runs callable synchronously, so one that calls an external/OS function cannot suspend and raises NotImplementedError. Same limitation as map/filter/sorted(key=...).

  • iter(callable, sentinel) compares result == sentinel, where CPython compares sentinel == result; only observable if the two sides have asymmetric __eq__.

  • A StopIteration raised by callable propagates; CPython treats it as clean exhaustion and stops iterating.

  • The vendored type stub is upstream typeshed’s verbatim, so -t accepts iter(obj) for an object with only __getitem__; Monty has no __getitem__ iteration fallback and raises TypeError at runtime.

  • -t accepts for x in obj (though not a, b = obj) for a class that opts out of iteration with __iter__ = None, which raises TypeError at runtime as it does in CPython.

  • Built-in iterators do not expose their dunders as attributes: hasattr(iter([1]), '__iter__') is False, where CPython reports True. Iteration itself works; only attribute lookup of the dunder differs. This covers every built-in iterator, including the itertools adaptors.

  • Iterator-specific attributes such as __length_hint__ are not exposed.