CLN: Orderdict -> defaultdict(list) (#30468)

Author: ShaharNaveh

pandas/core/groupby/generic.py

@@ -5,7 +5,7 @@
 These are user facing as the result of the ``df.groupby(...)`` operations,
 which here returns a DataFrameGroupBy object.
 """
-from collections import OrderedDict, abc, namedtuple
+from collections import OrderedDict, abc, defaultdict, namedtuple
 import copy
 from functools import partial
 from textwrap import dedent
@@ -1896,20 +1896,15 @@ def _normalize_keyword_aggregation(kwargs):
     """
     # Normalize the aggregation functions as Mapping[column, List[func]],
     # process normally, then fixup the names.
-    # TODO(Py35): When we drop python 3.5, change this to
-    # defaultdict(list)
     # TODO: aggspec type: typing.OrderedDict[str, List[AggScalar]]
     # May be hitting https://github.com/python/mypy/issues/5958
     # saying it doesn't have an attribute __name__
-    aggspec = OrderedDict()
+    aggspec = defaultdict(list)
     order = []
     columns, pairs = list(zip(*kwargs.items()))
 
     for name, (column, aggfunc) in zip(columns, pairs):
-        if column in aggspec:
-            aggspec[column].append(aggfunc)
-        else:
-            aggspec[column] = [aggfunc]
+        aggspec[column].append(aggfunc)
         order.append((column, com.get_callable_name(aggfunc) or aggfunc))
 
     # uniquify aggfunc name if duplicated in order list