hjtran commented on code in PR #35874:
URL: https://github.com/apache/beam/pull/35874#discussion_r2279560154
##########
sdks/python/apache_beam/transforms/core.py:
##########
@@ -3849,6 +3849,15 @@ def _extract_input_pvalues(self, pvalueish):
raise ValueError(
'Input to Flatten must be an iterable. '
'Got a value of type %s instead.' % type(pvalueish))
+
+ # Spot check to see if any of the items are iterables of PCollections
+ # and raise an error if so. This is always a user-error
+ for idx, item in enumerate(pvalueish):
+ if (isinstance(item, typing.Sequence) and len(item) and
+ isinstance(item[0], pvalue.PCollection)):
+ raise TypeError(
+ 'Inputs to Flatten cannot include an iterable of PCollections. '
+ f'(input at index {idx}: "{item}")')
Review Comment:
I intentionally only chose to sample the first element to save time in case
there were any especially large sequences and I figure most actual scenarios
where this happens it'll be a homogenous list.
@liferoad what do you think, is it worth checking the entire sequence?
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