Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the master branch of pandas.

Reproducible Example

import pandas as pd
df = pd.DataFrame({'key': ['z', 'z'], 'a': [1,2], 'b': [3,4]})
def f(x):
    print(x)
    return x.sum()
df.groupby('key').transform(f)

Issue Description

For every group the user function is first called with every series of this group (which is correct), but then with the group as a whole (which is not right, as the result is not used anywhere at all).

After digging through the source code I've found remnants of the 'fast path' and 'slow path' – an optimization that has long been gone, but those extra calls to user function are still there.

The commit which obliterates this optimization is
b8b6471
ENH: Add numba engine to groupby.transform (#32854)

After it was merged in, the path output variable of the _choose_path is not used anywhere any longer. So that extra call to the user function that was only necessary setup the path is not necessary as well:

-        path = None
         for name, group in gen:
             object.__setattr__(group, "name", name)

-            if path is None:
+            if engine == "numba":
+                values, index = split_for_numba(group)
+                res = numba_func(values, index, *args)
+                if func not in self._numba_func_cache:
+                    self._numba_func_cache[func] = numba_func
+                # Return the result as a DataFrame for concatenation later
+                res = DataFrame(res, index=group.index, columns=group.columns)
+            else:
                 # Try slow path and fast path.
                 try:
                     path, res = self._choose_path(fast_path, slow_path, group)
@@ -1376,8 +1422,6 @@ class DataFrameGroupBy(GroupBy[DataFrame]):
                 except ValueError as err:
                     msg = "transform must return a scalar value for each group"
                     raise ValueError(msg) from err
-            else:
-                res = path(group)

Or maybe it was done by mistake and deletion of the res=path(group) line should be reverted.

After this commit this line from the docs is no longer valid:

If f also supports application to the entire subframe, then a fast path is used starting from the second chunk.

Expected Behavior

0 1 # <-- this is correct
1 2
Name: a, dtype: int64
0 3 # <-- this is correct
1 4
Name: b, dtype: int64
a b # <-- this is wrong (result is ignored)
0 1 3
1 2 4
a b # <-- the result is correct
0 3 7
1 3 7

Installed Versions