TST: assert that informative error is raised when offset not supported as a period frequency is passed to DataFrame.asfreq (#56758)

* style

* add test

* pytest.raises

* freqstr

* add test for offsets

* rearrange per comment

* fixup

* fixup

---------

Co-authored-by: MarcoGorelli <[email protected]>

Author: aram-cedarwood

pandas/tests/frame/methods/test_asfreq.py

@@ -8,6 +8,7 @@
 from pandas import (
     DataFrame,
     DatetimeIndex,
+    PeriodIndex,
     Series,
     date_range,
     period_range,
@@ -257,3 +258,28 @@ def test_asfreq_frequency_M_Q_Y_A_deprecated(self, freq, freq_depr):
         with tm.assert_produces_warning(FutureWarning, match=depr_msg):
             result = df.asfreq(freq=freq_depr)
         tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "freq, error_msg",
+        [
+            (
+                "2MS",
+                "MS is not supported as period frequency",
+            ),
+            (
+                offsets.MonthBegin(),
+                r"\<MonthBegin\> is not supported as period frequency",
+            ),
+            (
+                offsets.DateOffset(months=2),
+                r"\<DateOffset: months=2\> is not supported as period frequency",
+            ),
+        ],
+    )
+    def test_asfreq_unsupported_freq(self, freq, error_msg):
+        # https://github.com/pandas-dev/pandas/issues/56718
+        index = PeriodIndex(["2020-01-01", "2021-01-01"], freq="M")
+        df = DataFrame({"a": Series([0, 1], index=index)})
+
+        with pytest.raises(ValueError, match=error_msg):
+            df.asfreq(freq=freq)