BUG/ENH: Fix apply to only call func once on the first column/row

doc/source/whatsnew/v1.1.0.rst

@@ -630,6 +630,45 @@ Using :meth:`DataFrame.groupby` with ``as_index=False`` and the function ``idxma
 
    df.groupby("a", as_index=False).nunique()
 
+.. _whatsnew_110.api_breaking.apply_applymap_first_once:
+
+apply and applymap on ``DataFrame`` evaluates first row/column only once
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. ipython:: python
+
+    df = pd.DataFrame({'a': [1, 2], 'b': [3, 6]})
+
+    def func(row):
+        print(row)
+        return row
+
+*Previous behavior*:
+
+.. code-block:: ipython
+
+    In [4]: df.apply(func, axis=1)
+    a    1
+    b    3
+    Name: 0, dtype: int64
+    a    1
+    b    3
+    Name: 0, dtype: int64
+    a    2
+    b    6
+    Name: 1, dtype: int64
+    Out[4]:
+       a  b
+    0  1  3
+    1  2  6
+
+*New behavior*:
+
+.. ipython:: python
+
+    df.apply(func, axis=1)
+
+
 .. _whatsnew_110.deprecations:
 
 Deprecations

pandas/_libs/reduction.pyx

@@ -107,6 +107,7 @@ cdef class Reducer:
 
         result = np.empty(self.nresults, dtype='O')
         it = <flatiter>PyArray_IterNew(result)
+        reduction_success = True
 
         try:
             for i in range(self.nresults):
@@ -134,21 +135,35 @@ cdef class Reducer:
                     res = self.f(chunk)
 
                 # TODO: reason for not squeezing here?
-                res = _extract_result(res, squeeze=False)
+                extracted_res = _extract_result(res, squeeze=False)
                 if i == 0:
                     # On the first pass, we check the output shape to see
                     #  if this looks like a reduction.
-                    _check_result_array(res, len(self.dummy))
-
-                PyArray_SETITEM(result, PyArray_ITER_DATA(it), res)
+                    #  If it does not, return the computed value to be used by the
+                    #  pure python implementation,
+                    #  so the function won't be called twice on the same object,
+                    #  and side effects would occur twice
+                    try:
+                        _check_result_array(extracted_res, len(self.dummy))
+                    except ValueError as err:
+                        if "Function does not reduce" not in str(err):
+                            # catch only the specific exception
+                            raise
+
+                        reduction_success = False
+                        PyArray_SETITEM(result, PyArray_ITER_DATA(it), copy(res))
+                        break
+
+                PyArray_SETITEM(result, PyArray_ITER_DATA(it), extracted_res)
                 chunk.data = chunk.data + self.increment
                 PyArray_ITER_NEXT(it)
+
         finally:
             # so we don't free the wrong memory
             chunk.data = dummy_buf
 
         result = maybe_convert_objects(result)
-        return result
+        return result, reduction_success
 
 
 cdef class _BaseGrouper:

pandas/core/apply.py

@@ -220,14 +220,12 @@ def apply_empty_result(self):
 
     def apply_raw(self):
         """ apply to the values as a numpy array """
-        try:
-            result = libreduction.compute_reduction(self.values, self.f, axis=self.axis)
-        except ValueError as err:
-            if "Function does not reduce" not in str(err):
-                # catch only ValueError raised intentionally in libreduction
-                raise
-            # We expect np.apply_along_axis to give a two-dimensional result, or
-            #  also raise.
+        result, reduction_success = libreduction.compute_reduction(
+            self.values, self.f, axis=self.axis
+        )
+
+        # We expect np.apply_along_axis to give a two-dimensional result, or raise.
+        if not reduction_success:
             result = np.apply_along_axis(self.f, self.axis, self.values)
 
         # TODO: mixed type case
@@ -265,6 +263,9 @@ def apply_broadcast(self, target: "DataFrame") -> "DataFrame":
 
     def apply_standard(self):
 
+        # partial result that may be returned from reduction
+        partial_result = None
+
         # try to reduce first (by default)
         # this only matters if the reduction in values is of different dtype
         # e.g. if we want to apply to a SparseFrame, then can't directly reduce
@@ -292,13 +293,9 @@ def apply_standard(self):
             )
 
             try:
-                result = libreduction.compute_reduction(
+                result, reduction_success = libreduction.compute_reduction(
                     values, self.f, axis=self.axis, dummy=dummy, labels=labels
                 )
-            except ValueError as err:
-                if "Function does not reduce" not in str(err):
-                    # catch only ValueError raised intentionally in libreduction
-                    raise
             except TypeError:
                 # e.g. test_apply_ignore_failures we just ignore
                 if not self.ignore_failures:
@@ -307,39 +304,53 @@ def apply_standard(self):
                 # reached via numexpr; fall back to python implementation
                 pass
             else:
-                return self.obj._constructor_sliced(result, index=labels)
+                if reduction_success:
+                    return self.obj._constructor_sliced(result, index=labels)
 
-        # compute the result using the series generator
-        results, res_index = self.apply_series_generator()
+                # no exceptions - however reduction was unsuccessful,
+                # use the computed function result for first element
+                partial_result = result[0]
+                if isinstance(partial_result, ABCSeries):
+                    partial_result = partial_result.infer_objects()
+
+        # compute the result using the series generator,
+        # use the result computed while trying to reduce if available.
+        results, res_index = self.apply_series_generator(partial_result)
 
         # wrap results
         return self.wrap_results(results, res_index)
 
-    def apply_series_generator(self) -> Tuple[ResType, "Index"]:
+    def apply_series_generator(self, partial_result=None) -> Tuple[ResType, "Index"]:
         series_gen = self.series_generator
         res_index = self.result_index
 
-        keys = []
         results = {}
+
+        # If a partial result was already computed,
+        # use it instead of running on the first element again
+        series_gen_enumeration = enumerate(series_gen)
+        if partial_result is not None:
+            i, v = next(series_gen_enumeration)
+            results[i] = partial_result
+
         if self.ignore_failures:
             successes = []
-            for i, v in enumerate(series_gen):
+            for i, v in series_gen_enumeration:
                 try:
                     results[i] = self.f(v)
                 except Exception:
                     pass
                 else:
-                    keys.append(v.name)
                     successes.append(i)
 
             # so will work with MultiIndex
             if len(successes) < len(res_index):
                 res_index = res_index.take(successes)
 
         else:
-            for i, v in enumerate(series_gen):
+            for i, v in series_gen_enumeration:
+
                 results[i] = self.f(v)
-                keys.append(v.name)
 
         return results, res_index
 

pandas/core/frame.py

@@ -7421,14 +7421,6 @@ def applymap(self, func) -> "DataFrame":
         --------
         DataFrame.apply : Apply a function along input axis of DataFrame.
 
-        Notes
-        -----
-        In the current implementation applymap calls `func` twice on the
-        first column/row to decide whether it can take a fast or slow
-        code path. This can lead to unexpected behavior if `func` has
-        side-effects, as they will take effect twice for the first
-        column/row.
-
         Examples
         --------
         >>> df = pd.DataFrame([[1, 2.12], [3.356, 4.567]])

pandas/tests/frame/test_apply.py

@@ -718,12 +718,73 @@ def apply_list(row):
 
     def test_apply_noreduction_tzaware_object(self):
         # https://github.com/pandas-dev/pandas/issues/31505
-        df = pd.DataFrame({"foo": [pd.Timestamp("2020", tz="UTC")]}, dtype="object")
+        df = pd.DataFrame(
+            {"foo": [pd.Timestamp("2020", tz="UTC")]}, dtype="datetime64[ns, UTC]"
+        )
         result = df.apply(lambda x: x)
         tm.assert_frame_equal(result, df)
         result = df.apply(lambda x: x.copy())
         tm.assert_frame_equal(result, df)
 
+    def test_apply_function_runs_once(self):
+        # https://github.com/pandas-dev/pandas/issues/30815
+
+        df = pd.DataFrame({"a": [1, 2, 3]})
+        names = []  # Save row names function is applied to
+
+        def reducing_function(row):
+            names.append(row.name)
+
+        def non_reducing_function(row):
+            names.append(row.name)
+            return row
+
+        for func in [reducing_function, non_reducing_function]:
+            del names[:]
+
+            df.apply(func, axis=1)
+            assert names == list(df.index)
+
+    @pytest.mark.xfail(
+        reason="The 'run once' enhancement for apply_raw not implemented yet."
+    )
+    def test_apply_raw_function_runs_once(self):
+        # https://github.com/pandas-dev/pandas/issues/34506
+
+        df = pd.DataFrame({"a": [1, 2, 3]})
+        values = []  # Save row values function is applied to
+
+        def reducing_function(row):
+            values.extend(row)
+
+        def non_reducing_function(row):
+            values.extend(row)
+            return row
+
+        for func in [reducing_function, non_reducing_function]:
+            del values[:]
+
+            df.apply(func, raw=True, axis=1)
+            assert values == list(df.a.to_list())
+
+    def test_applymap_function_runs_once(self):
+
+        df = pd.DataFrame({"a": [1, 2, 3]})
+        values = []  # Save values function is applied to
+
+        def reducing_function(val):
+            values.append(val)
+
+        def non_reducing_function(val):
+            values.append(val)
+            return val
+
+        for func in [reducing_function, non_reducing_function]:
+            del values[:]
+
+            df.applymap(func)
+            assert values == df.a.to_list()
+
 
 class TestInferOutputShape:
     # the user has supplied an opaque UDF where

pandas/tests/groupby/test_bin_groupby.py

@@ -153,20 +153,20 @@ def test_int_index(self):
             )
 
         dummy = Series(0.0, index=np.arange(100))
-        result = libreduction.compute_reduction(
+        result, _ = libreduction.compute_reduction(
             arr, np.sum, dummy=dummy, labels=Index(np.arange(4))
         )
         expected = arr.sum(0)
         tm.assert_almost_equal(result, expected)
 
         dummy = Series(0.0, index=np.arange(4))
-        result = libreduction.compute_reduction(
+        result, _ = libreduction.compute_reduction(
             arr, np.sum, axis=1, dummy=dummy, labels=Index(np.arange(100))
         )
         expected = arr.sum(1)
         tm.assert_almost_equal(result, expected)
 
-        result = libreduction.compute_reduction(
+        result, _ = libreduction.compute_reduction(
             arr, np.sum, axis=1, dummy=dummy, labels=Index(np.arange(100))
         )
         tm.assert_almost_equal(result, expected)