[jayyyyqwq]

SlidingWindows.__init__ in the Python SDK checks that size is positive but never checks period.

With period=0 the constructor fails on Timestamp.of(offset) % period with ZeroDivisionError: integer modulo by zero (window.py:479 before this change). With a negative period, for example SlidingWindows(size=10, period=-5), it constructs fine and assign() returns [], because the range(...) in assign steps the wrong way. A pipeline using it drops every element without any error.

FixedWindows(size=0) and Sessions(gap_size=0) already raise ValueError('The size parameter must be strictly positive.'), and the Java SlidingWindows rejects this through its 0 <= offset < period check. I added the matching check for period right after the size check:

if period <= 0:
  raise ValueError('The period parameter must be strictly positive.')

Testing: I added test_sliding_windows_non_positive_period to window_test.py. Before the change it fails with ZeroDivisionError: integer modulo by zero at window.py:479. It also asserts that FixedWindows(size=0) raises ValueError, which passes before and after. After the change window_test.py passes (20 passed), and the sliding or window tests in sideinputs_test.py and trigger_manager_test.py pass (12 passed). In the repo, yaml_provider.py and the Nexmark queries pass period from user or config values. A zero period already failed there, and a negative one produced no windows, so it now fails with the clear error. I did not run combiners_test.py, util_test.py or pipeline_test.py locally, since my venv lacks pyhamcrest and mock, and I did not run the full CI.

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📋 Review Summary

This PR adds validation to SlidingWindows.__init__ to ensure the period parameter is strictly positive, preventing ZeroDivisionError when period == 0 and silent element dropping when period < 0. The implementation and accompanying unit test are clean, accurate, and consistent with existing parameter validation in FixedWindows and Sessions.

🔍 General Feedback

• The validation logic and error message match existing conventions across apache_beam/transforms/window.py.
• Unit tests in window_test.py appropriately cover both zero and negative period values.

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[github-actions]

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