BUG: clean_fill_method failing to raise (pandas-dev#53620)

* BUG: clean_fill_method failing to raise

* mypy fixup

* Update doc/source/whatsnew/v2.1.0.rst

Co-authored-by: Matthew Roeschke <[email protected]>

* sort note

---------

Co-authored-by: Matthew Roeschke <[email protected]>

doc/source/whatsnew/v2.1.0.rst

@@ -506,9 +506,11 @@ Other
 - Bug in :func:`assert_almost_equal` now throwing assertion error for two unequal sets (:issue:`51727`)
 - Bug in :func:`assert_frame_equal` checks category dtypes even when asked not to check index type (:issue:`52126`)
 - Bug in :meth:`DataFrame.reindex` with a ``fill_value`` that should be inferred with a :class:`ExtensionDtype` incorrectly inferring ``object`` dtype (:issue:`52586`)
+- Bug in :meth:`Series.align`, :meth:`DataFrame.align`, :meth:`Series.reindex`, :meth:`DataFrame.reindex`, :meth:`Series.interpolate`, :meth:`DataFrame.interpolate`, incorrectly failing to raise with method="asfreq" (:issue:`53620`)
 - Bug in :meth:`Series.map` when giving a callable to an empty series, the returned series had ``object`` dtype. It now keeps the original dtype (:issue:`52384`)
 - Bug in :meth:`Series.memory_usage` when ``deep=True`` throw an error with Series of objects and the returned value is incorrect, as it does not take into account GC corrections (:issue:`51858`)
 - Fixed incorrect ``__name__`` attribute of ``pandas._libs.json`` (:issue:`52898`)
+-
 
 .. ***DO NOT USE THIS SECTION***
 

pandas/core/generic.py

@@ -9681,7 +9681,9 @@ def align(
             method = None
         if limit is lib.no_default:
             limit = None
-        method = clean_fill_method(method)
+
+        if method is not None:
+            method = clean_fill_method(method)
 
         if broadcast_axis is not lib.no_default:
             # GH#51856

pandas/core/internals/blocks.py

@@ -1369,6 +1369,12 @@ def interpolate(
             m = missing.clean_fill_method(method)
         except ValueError:
             m = None
+            # error: Non-overlapping equality check (left operand type:
+            # "Literal['backfill', 'bfill', 'ffill', 'pad']", right
+            # operand type: "Literal['asfreq']")
+            if method == "asfreq":  # type: ignore[comparison-overlap]
+                # clean_fill_method used to allow this
+                raise
         if m is None and self.dtype.kind != "f":
             # only deal with floats
             # bc we already checked that can_hold_na, we don't have int dtype here

pandas/core/missing.py

@@ -124,11 +124,7 @@ def mask_missing(arr: ArrayLike, values_to_mask) -> npt.NDArray[np.bool_]:
     return mask
 
 
-def clean_fill_method(method: str | None, allow_nearest: bool = False):
-    # asfreq is compat for resampling
-    if method in [None, "asfreq"]:
-        return None
-
+def clean_fill_method(method: str, allow_nearest: bool = False):
     if isinstance(method, str):
         method = method.lower()
         if method == "ffill":
@@ -1006,6 +1002,8 @@ def get_fill_func(method, ndim: int = 1):
 
 
 def clean_reindex_fill_method(method) -> ReindexMethod | None:
+    if method is None:
+        return None
     return clean_fill_method(method, allow_nearest=True)
 
 

pandas/core/resample.py

@@ -1506,6 +1506,8 @@ def _upsample(self, method, limit: int | None = None, fill_value=None):
             result = obj.copy()
             result.index = res_index
         else:
+            if method == "asfreq":
+                method = None
             result = obj.reindex(
                 res_index, method=method, limit=limit, fill_value=fill_value
             )
@@ -1624,6 +1626,8 @@ def _upsample(self, method, limit: int | None = None, fill_value=None):
         memb = ax.asfreq(self.freq, how=self.convention)
 
         # Get the fill indexer
+        if method == "asfreq":
+            method = None
         indexer = memb.get_indexer(new_index, method=method, limit=limit)
         new_obj = _take_new_index(
             obj,

pandas/tests/copy_view/test_interp_fillna.py

@@ -113,7 +113,7 @@ def test_interpolate_cleaned_fill_method(using_copy_on_write):
     df = DataFrame({"a": ["a", np.nan, "c"], "b": 1})
     df_orig = df.copy()
 
-    result = df.interpolate(method="asfreq")
+    result = df.interpolate(method="linear")
 
     if using_copy_on_write:
         assert np.shares_memory(get_array(result, "a"), get_array(df, "a"))

pandas/tests/frame/methods/test_align.py

@@ -14,6 +14,14 @@
 
 
 class TestDataFrameAlign:
+    def test_align_asfreq_method_raises(self):
+        df = DataFrame({"A": [1, np.nan, 2]})
+        msg = "Invalid fill method"
+        msg2 = "The 'method', 'limit', and 'fill_axis' keywords"
+        with pytest.raises(ValueError, match=msg):
+            with tm.assert_produces_warning(FutureWarning, match=msg2):
+                df.align(df.iloc[::-1], method="asfreq")
+
     def test_frame_align_aware(self):
         idx1 = date_range("2001", periods=5, freq="H", tz="US/Eastern")
         idx2 = date_range("2001", periods=5, freq="2H", tz="US/Eastern")

pandas/tests/frame/methods/test_reindex.py

@@ -1291,3 +1291,10 @@ def test_reindex_not_category(self, index_df, index_res, index_exp):
         result = df.reindex(index=index_res)
         expected = DataFrame(index=index_exp)
         tm.assert_frame_equal(result, expected)
+
+    def test_invalid_method(self):
+        df = DataFrame({"A": [1, np.nan, 2]})
+
+        msg = "Invalid fill method"
+        with pytest.raises(ValueError, match=msg):
+            df.reindex([1, 0, 2], method="asfreq")

pandas/tests/series/methods/test_interpolate.py

@@ -845,3 +845,9 @@ def test_interpolate_unsorted_index(self, ascending, expected_values):
         result = ts.sort_index(ascending=ascending).interpolate(method="index")
         expected = Series(data=expected_values, index=expected_values, dtype=float)
         tm.assert_series_equal(result, expected)
+
+    def test_interpolate_afreq_raises(self):
+        ser = Series(["a", None, "b"], dtype=object)
+        msg = "Invalid fill method"
+        with pytest.raises(ValueError, match=msg):
+            ser.interpolate(method="asfreq")