BUG/CLN: Decouple Series/DataFrame.transform (pandas-dev#35964)

doc/source/whatsnew/v1.2.0.rst

@@ -344,6 +344,7 @@ Other
 ^^^^^
 - Bug in :meth:`DataFrame.replace` and :meth:`Series.replace` incorrectly raising ``AssertionError`` instead of ``ValueError`` when invalid parameter combinations are passed (:issue:`36045`)
 - Bug in :meth:`DataFrame.replace` and :meth:`Series.replace` with numeric values and string ``to_replace`` (:issue:`34789`)
+- Bug in :meth:`Series.transform` would give incorrect results or raise when the argument ``func`` was dictionary (:issue:`35811`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/aggregation.py

@@ -18,9 +18,10 @@
     Union,
 )
 
-from pandas._typing import AggFuncType, FrameOrSeries, Label
+from pandas._typing import AggFuncType, Axis, FrameOrSeries, Label
 
 from pandas.core.dtypes.common import is_dict_like, is_list_like
+from pandas.core.dtypes.generic import ABCDataFrame, ABCSeries
 
 from pandas.core.base import SpecificationError
 import pandas.core.common as com
@@ -384,3 +385,98 @@ def validate_func_kwargs(
     if not columns:
         raise TypeError(no_arg_message)
     return columns, func
+
+
+def transform(
+    obj: FrameOrSeries, func: AggFuncType, axis: Axis, *args, **kwargs,
+) -> FrameOrSeries:
+    """
+    Transform a DataFrame or Series
+
+    Parameters
+    ----------
+    obj : DataFrame or Series
+        Object to compute the transform on.
+    func : string, function, list, or dictionary
+        Function(s) to compute the transform with.
+    axis : {0 or 'index', 1 or 'columns'}
+        Axis along which the function is applied:
+
+        * 0 or 'index': apply function to each column.
+        * 1 or 'columns': apply function to each row.
+
+    Returns
+    -------
+    DataFrame or Series
+        Result of applying ``func`` along the given axis of the
+        Series or DataFrame.
+
+    Raises
+    ------
+    ValueError
+        If the transform function fails or does not transform.
+    """
+    from pandas.core.reshape.concat import concat
+
+    is_series = obj.ndim == 1
+
+    if obj._get_axis_number(axis) == 1:
+        assert not is_series
+        return transform(obj.T, func, 0, *args, **kwargs).T
+
+    if isinstance(func, list):
+        if is_series:
+            func = {com.get_callable_name(v) or v: v for v in func}
+        else:
+            func = {col: func for col in obj}
+
+    if isinstance(func, dict):
+        if not is_series:
+            cols = sorted(set(func.keys()) - set(obj.columns))
+            if len(cols) > 0:
+                raise SpecificationError(f"Column(s) {cols} do not exist")
+
+        if any(isinstance(v, dict) for v in func.values()):
+            # GH 15931 - deprecation of renaming keys
+            raise SpecificationError("nested renamer is not supported")
+
+        results = {}
+        for name, how in func.items():
+            colg = obj._gotitem(name, ndim=1)
+            try:
+                results[name] = transform(colg, how, 0, *args, **kwargs)
+            except Exception as e:
+                if str(e) == "Function did not transform":
+                    raise e
+
+        # combine results
+        if len(results) == 0:
+            raise ValueError("Transform function failed")
+        return concat(results, axis=1)
+
+    # func is either str or callable
+    try:
+        if isinstance(func, str):
+            result = obj._try_aggregate_string_function(func, *args, **kwargs)
+        else:
+            f = obj._get_cython_func(func)
+            if f and not args and not kwargs:
+                result = getattr(obj, f)()
+            else:
+                try:
+                    result = obj.apply(func, args=args, **kwargs)
+                except Exception:
+                    result = func(obj, *args, **kwargs)
+    except Exception:
+        raise ValueError("Transform function failed")
+
+    # Functions that transform may return empty Series/DataFrame
+    # when the dtype is not appropriate
+    if isinstance(result, (ABCSeries, ABCDataFrame)) and result.empty:
+        raise ValueError("Transform function failed")
+    if not isinstance(result, (ABCSeries, ABCDataFrame)) or not result.index.equals(
+        obj.index
+    ):
+        raise ValueError("Function did not transform")
+
+    return result

pandas/core/base.py

@@ -4,7 +4,7 @@
 
 import builtins
 import textwrap
-from typing import Any, Dict, FrozenSet, List, Optional, Union
+from typing import Any, Callable, Dict, FrozenSet, List, Optional, Union
 
 import numpy as np
 
@@ -560,7 +560,7 @@ def _aggregate_multiple_funcs(self, arg, _axis):
                 ) from err
             return result
 
-    def _get_cython_func(self, arg: str) -> Optional[str]:
+    def _get_cython_func(self, arg: Callable) -> Optional[str]:
         """
         if we define an internal function for this argument, return it
         """

pandas/core/frame.py

@@ -45,6 +45,7 @@
 from pandas._libs import algos as libalgos, lib, properties
 from pandas._libs.lib import no_default
 from pandas._typing import (
+    AggFuncType,
     ArrayLike,
     Axes,
     Axis,
@@ -116,7 +117,7 @@
 
 from pandas.core import algorithms, common as com, nanops, ops
 from pandas.core.accessor import CachedAccessor
-from pandas.core.aggregation import reconstruct_func, relabel_result
+from pandas.core.aggregation import reconstruct_func, relabel_result, transform
 from pandas.core.arrays import Categorical, ExtensionArray
 from pandas.core.arrays.datetimelike import DatetimeLikeArrayMixin as DatetimeLikeArray
 from pandas.core.arrays.sparse import SparseFrameAccessor
@@ -7462,15 +7463,16 @@ def _aggregate(self, arg, axis=0, *args, **kwargs):
     agg = aggregate
 
     @doc(
-        NDFrame.transform,
+        _shared_docs["transform"],
         klass=_shared_doc_kwargs["klass"],
         axis=_shared_doc_kwargs["axis"],
     )
-    def transform(self, func, axis=0, *args, **kwargs) -> DataFrame:
-        axis = self._get_axis_number(axis)
-        if axis == 1:
-            return self.T.transform(func, *args, **kwargs).T
-        return super().transform(func, *args, **kwargs)
+    def transform(
+        self, func: AggFuncType, axis: Axis = 0, *args, **kwargs
+    ) -> DataFrame:
+        result = transform(self, func, axis, *args, **kwargs)
+        assert isinstance(result, DataFrame)
+        return result
 
     def apply(self, func, axis=0, raw=False, result_type=None, args=(), **kwds):
         """

pandas/core/generic.py

@@ -10648,80 +10648,6 @@ def ewm(
             times=times,
         )
 
-    @doc(klass=_shared_doc_kwargs["klass"], axis="")
-    def transform(self, func, *args, **kwargs):
-        """
-        Call ``func`` on self producing a {klass} with transformed values.
-
-        Produced {klass} will have same axis length as self.
-
-        Parameters
-        ----------
-        func : function, str, list or dict
-            Function to use for transforming the data. If a function, must either
-            work when passed a {klass} or when passed to {klass}.apply.
-
-            Accepted combinations are:
-
-            - function
-            - string function name
-            - list of functions and/or function names, e.g. ``[np.exp, 'sqrt']``
-            - dict of axis labels -> functions, function names or list of such.
-        {axis}
-        *args
-            Positional arguments to pass to `func`.
-        **kwargs
-            Keyword arguments to pass to `func`.
-
-        Returns
-        -------
-        {klass}
-            A {klass} that must have the same length as self.
-
-        Raises
-        ------
-        ValueError : If the returned {klass} has a different length than self.
-
-        See Also
-        --------
-        {klass}.agg : Only perform aggregating type operations.
-        {klass}.apply : Invoke function on a {klass}.
-
-        Examples
-        --------
-        >>> df = pd.DataFrame({{'A': range(3), 'B': range(1, 4)}})
-        >>> df
-           A  B
-        0  0  1
-        1  1  2
-        2  2  3
-        >>> df.transform(lambda x: x + 1)
-           A  B
-        0  1  2
-        1  2  3
-        2  3  4
-
-        Even though the resulting {klass} must have the same length as the
-        input {klass}, it is possible to provide several input functions:
-
-        >>> s = pd.Series(range(3))
-        >>> s
-        0    0
-        1    1
-        2    2
-        dtype: int64
-        >>> s.transform([np.sqrt, np.exp])
-               sqrt        exp
-        0  0.000000   1.000000
-        1  1.000000   2.718282
-        2  1.414214   7.389056
-        """
-        result = self.agg(func, *args, **kwargs)
-        if is_scalar(result) or len(result) != len(self):
-            raise ValueError("transforms cannot produce aggregated results")
-
-        return result
-
     # ----------------------------------------------------------------------
     # Misc methods
 

pandas/core/series.py

@@ -25,6 +25,7 @@
 from pandas._libs import lib, properties, reshape, tslibs
 from pandas._libs.lib import no_default
 from pandas._typing import (
+    AggFuncType,
     ArrayLike,
     Axis,
     DtypeObj,
@@ -89,6 +90,7 @@
 from pandas.core.indexes.timedeltas import TimedeltaIndex
 from pandas.core.indexing import check_bool_indexer
 from pandas.core.internals import SingleBlockManager
+from pandas.core.shared_docs import _shared_docs
 from pandas.core.sorting import ensure_key_mapped
 from pandas.core.strings import StringMethods
 from pandas.core.tools.datetimes import to_datetime
@@ -4081,14 +4083,16 @@ def aggregate(self, func=None, axis=0, *args, **kwargs):
     agg = aggregate
 
     @doc(
-        NDFrame.transform,
+        _shared_docs["transform"],
         klass=_shared_doc_kwargs["klass"],
         axis=_shared_doc_kwargs["axis"],
     )
-    def transform(self, func, axis=0, *args, **kwargs):
-        # Validate the axis parameter
-        self._get_axis_number(axis)
-        return super().transform(func, *args, **kwargs)
+    def transform(
+        self, func: AggFuncType, axis: Axis = 0, *args, **kwargs
+    ) -> FrameOrSeriesUnion:
+        from pandas.core.aggregation import transform
+
+        return transform(self, func, axis, *args, **kwargs)
 
     def apply(self, func, convert_dtype=True, args=(), **kwds):
         """

pandas/core/shared_docs.py

@@ -257,3 +257,72 @@
 1      b          B          E      3
 2      c          B          E      5
 """
+
+_shared_docs[
+    "transform"
+] = """\
+Call ``func`` on self producing a {klass} with transformed values.
+
+Produced {klass} will have same axis length as self.
+
+Parameters
+----------
+func : function, str, list or dict
+    Function to use for transforming the data. If a function, must either
+    work when passed a {klass} or when passed to {klass}.apply.
+
+    Accepted combinations are:
+
+    - function
+    - string function name
+    - list of functions and/or function names, e.g. ``[np.exp, 'sqrt']``
+    - dict of axis labels -> functions, function names or list of such.
+{axis}
+*args
+    Positional arguments to pass to `func`.
+**kwargs
+    Keyword arguments to pass to `func`.
+
+Returns
+-------
+{klass}
+    A {klass} that must have the same length as self.
+
+Raises
+------
+ValueError : If the returned {klass} has a different length than self.
+
+See Also
+--------
+{klass}.agg : Only perform aggregating type operations.
+{klass}.apply : Invoke function on a {klass}.
+
+Examples
+--------
+>>> df = pd.DataFrame({{'A': range(3), 'B': range(1, 4)}})
+>>> df
+   A  B
+0  0  1
+1  1  2
+2  2  3
+>>> df.transform(lambda x: x + 1)
+   A  B
+0  1  2
+1  2  3
+2  3  4
+
+Even though the resulting {klass} must have the same length as the
+input {klass}, it is possible to provide several input functions:
+
+>>> s = pd.Series(range(3))
+>>> s
+0    0
+1    1
+2    2
+dtype: int64
+>>> s.transform([np.sqrt, np.exp])
+       sqrt        exp
+0  0.000000   1.000000
+1  1.000000   2.718282
+2  1.414214   7.389056
+"""