[Bug]: GenerateSequence Python SDK constructor does not support period parameter

[jaymitdp]

What happened?

In the Apache Beam Python SDK, the GenerateSequence transform’s constructor does not accept the period parameter, even though the Java SDK requires it when elements_per_period is specified.

This means that if you want to use elements_per_period to generate a rate-limited sequence, you cannot specify the accompanying period through the Python SDK. This mismatch leads to a runtime failure inside the Java expansion service with the error:

RuntimeError: java.lang.IllegalArgumentException: elements per period specified, but no period specified
...

This prevents users from correctly controlling the rate of elements generated by GenerateSequence from Python, making elements_per_period unusable without period.

Steps to Reproduce

1. In a Beam Python pipeline, attempt to instantiate GenerateSequence with elements_per_period set, e.g.:

GenerateSequence(start=0, elements_per_period=1)

2. Notice that you cannot pass period to the constructor since it is not accepted as an argument.

3. Run the pipeline on a runner using the expansion service (e.g., FlinkRunner).

4. Observe the runtime error about missing period.

Expected Behavior

• The Python SDK GenerateSequence constructor should support a period parameter.
• This would allow users to specify elements_per_period and period, aligning with the Java SDK and the underlying runner’s requirements.

Actual Behavior

• Python SDK constructor rejects period parameter (no support).
• Pipeline fails at runtime with an IllegalArgumentException from the Java expansion service due to missing period.

Environment

• Apache Beam Python SDK version: 2.65.0
• Runner: FlinkRunner (with expansion service)
• Java expansion service version: matches Beam 2.65.0

This gap between the Python SDK interface and the Java expansion service causes confusing runtime failures and limits the usability of GenerateSequence in Python for rate-limited sequences.

Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

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