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https://issues.apache.org/jira/browse/BEAM-8140?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16948881#comment-16948881
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Robert Bradshaw commented on BEAM-8140:
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I know we made it possible to apply the same PTransform as many times as you
want within a pipeline, but don't recall why it cares about (or stores a
reference to) the pipeline itself. This code was intended to prohibit mixing
values across pipelines (e.g. flatting a PCollection with one pipeline to a
PCollection of another). This should be fixed.
> Python API: PTransform should be immutable
> ------------------------------------------
>
> Key: BEAM-8140
> URL: https://issues.apache.org/jira/browse/BEAM-8140
> Project: Beam
> Issue Type: Improvement
> Components: sdk-py-core
> Reporter: Chris Suchandk
> Assignee: Robert Bradshaw
> Priority: Major
>
> While the Java API seems fine the Python API is (at least) counterintuitive.
> Let's see the following example:
> {code:python}
> p1 = beam.Pipeline()
> p2 = beam.Pipeline()
> node = 'ReadTrainData' >> beam.io.ReadFromText("/tmp/aaa.txt")
> p1 | node
> p2 | node //fails here {code}
> The code above will fail because the _node_ somehow remembers that it was
> already attached to _p1_. In fact, unlike in Java, the | (apply) method is
> defined on the _PTransform_.
> If any, only the pipeline object should be mutable here.
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