shoyer commented on a change in pull request #9168:
URL: https://github.com/apache/beam/pull/9168#discussion_r462035731
##########
File path: sdks/python/apache_beam/transforms/core.py
##########
@@ -1316,6 +1322,144 @@ def Map(fn, *args, **kwargs): # pylint:
disable=invalid-name
return pardo
+def MapTuple(fn, *args, **kwargs): # pylint: disable=invalid-name
+ r""":func:`MapTuple` is like :func:`Map` but expects tuple inputs and
+ flattens them into multiple input arguments.
+
+ beam.MapTuple(lambda a, b, ...: ...)
+
+ is equivalent to Python 2
+
+ beam.Map(lambda (a, b, ...), ...: ...)
+
+ In other words
+
+ beam.MapTuple(fn)
+
+ is equivalent to
+
+ beam.Map(lambda element, ...: fn(\*element, ...))
+
+ This can be useful when processing a PCollection of tuples
+ (e.g. key-value pairs).
+
+ Args:
+ fn (callable): a callable object.
+ *args: positional arguments passed to the transform callable.
+ **kwargs: keyword arguments passed to the transform callable.
+
+ Returns:
+ ~apache_beam.pvalue.PCollection:
+ A :class:`~apache_beam.pvalue.PCollection` containing the
+ :func:`MapTuple` outputs.
+
+ Raises:
+ ~exceptions.TypeError: If the **fn** passed as argument is not a callable.
+ Typical error is to pass a :class:`DoFn` instance which is supported only
+ for :class:`ParDo`.
+ """
+ if not callable(fn):
+ raise TypeError(
+ 'MapTuple can be used only with callable objects. '
+ 'Received %r instead.' % (fn))
+
+ label = 'MapTuple(%s)' % ptransform.label_from_callable(fn)
+
+ argspec = getfullargspec(fn)
+ num_defaults = len(argspec.defaults or ())
+ if num_defaults < len(args) + len(kwargs):
+ raise TypeError('Side inputs must have defaults for MapTuple.')
Review comment:
@robertwb I'm quite curious about the reason for this restriction,
which doesn't exist on `beam.Map`.
Is this just a detail of the current implementation that could potentially
removed, or is there a more fundamental reason for it?
Also, would keyword only arguments be OK on Python 3? Based on the
implementation, I would assume the answer is
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