[GitHub] [beam] rohdesamuel commented on a change in pull request #14922: [BEAM-9547] Add support for `DataFrame.value_counts()`

[GitBox <gi...@apache.org>]

rohdesamuel commented on a change in pull request #14922:
URL: https://github.com/apache/beam/pull/14922#discussion_r646776193



##########
File path: sdks/python/apache_beam/dataframe/frames.py
##########
@@ -458,6 +458,32 @@ def size(self):
               requires_partition_by=partitionings.Singleton(),
               preserves_partition_by=partitionings.Singleton()))
 
+  def length(self):
+    """Alternative to ``len(df)`` which returns a deferred result that can be
+    used in arithmetic with :class:`DeferredSeries` or
+    :class:`DeferredDataFrame` instances."""
+    lengths = expressions.ComputedExpression(
+        'get_lengths',
+        # Wrap scalar results in a Series for easier concatenation later
+        lambda df: pd.Series(len(df)),
+        [self._expr],
+        requires_partition_by=partitionings.Arbitrary(),
+        preserves_partition_by=partitionings.Singleton())
+
+    with expressions.allow_non_parallel_operations(True):
+      return frame_base.DeferredFrame.wrap(
+          expressions.ComputedExpression(
+              'sum_lengths',
+              lambda lengths: lengths.sum(), [lengths],
+              requires_partition_by=partitionings.Singleton(),
+              preserves_partition_by=partitionings.Singleton()))
+
+  def __len__(self):
+    raise frame_base.WontImplementError(
+        "len(df) is not currently supported because it produces a non-deferred "
+        "result. Consider using df.length() instead.",
+        reason="non-deferred-result")

Review comment:
       ohh that's really weird! Good to know though




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