tvalentyn commented on code in PR #24435:
URL: https://github.com/apache/beam/pull/24435#discussion_r1037798432
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sdks/python/apache_beam/transforms/core.py:
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@@ -2475,7 +2475,7 @@ def typed(transform):
def inject_default(_, combined):
if combined:
- assert len(combined) == 1
+ assert len(combined) >= 1
Review Comment:
the intent of the `_inject_default` function is to add a default element in
the combined result if the combined result is empty and a default value can be
well-defined as `combine_fn.apply([], *args, **kwargs)`.
Users can disable default values if they use `.without_defaults()` value
when they apply a combiner.
The replacement of the empty output with default value happens at the last
step of combining. At this stage, PCollection being combined should be reduced
to 1 element, or be empty. Default value is not necessary when the result of
combining is NOT empty - in this case we should take the reduced result.
However, since this is evaluated after prior combining, we expect that the
result of prior combining is a single value. Therefore, we have an assertion
that checks that prior combining has reduced the pcollection to 1 element. If
somehow we have more than 1 element, then the result may not be fully combined,
so we should understand how that happened.
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