itertools: defer dropwhile predicate validation - #8504
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Store the `dropwhile` predicate as a Python object and call it while advancing the iterator. This defers callable validation until the predicate is first needed, matching CPython for empty input while preserving exception propagation during iteration. Remove the now-passing `test_dropwhile` expected-failure marker. Assisted-by: Codex:gpt-5.6-sol
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📝 WalkthroughWalkthroughThe ChangesDropwhile predicate handling
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📦 Library DependenciesThe following Lib/ modules were modified. Here are their dependencies: [x] test: cpython/Lib/test/test_itertools.py (TODO: 4) dependencies: dependent tests: (56 tests)
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Summary
Defer
itertools.dropwhile()predicate validation until iteration, matchingCPython's lazy behavior.
Details
dropwhilestored its predicate asArgCallable, which rejected anon-callable predicate during construction. CPython permits construction and
only calls (and therefore validates) the predicate when it consumes an item.
This matters for an empty iterable and for the timing of exceptions.
Store the predicate as
PyObjectRefand invoke it fromnext(). The changekeeps the existing first-false-item behavior and exception propagation while
aligning validation timing with CPython.
TestBasicOps.test_dropwhilenow passes with its existing CPython test body,so remove its
expectedFailuremarker.Testing
uv tool run prek run --all-filescargo fmt --checkcargo run --release Lib/test/test_itertools.pycargo run --release -- -m test test_itertoolscargo run -- extra_tests/snippets/stdlib_itertools.pycargo clippy -p rustpython-vm --all-targets -- -Dwarningscargo test --workspace --exclude rustpython_wasm --exclude rustpython-venvlauncher --exclude rustpython-capigit diff --checkAI assistance: Codex:gpt-5.6-sol
Summary by CodeRabbit
dropwhilepredicate handling for more reliable iterator behavior.