BUG: incorrect output of first('1M') in case if first index is the last day of the month (#29623)

pandas/core/generic.py

@@ -8550,6 +8550,11 @@ def first(self, offset):
             return self
 
         offset = to_offset(offset)
+
+        # Check if offset and index[0] overlap
+        if offset.onOffset(self.index[0]):
+            return self.loc[:self.index[0]]
+
         end_date = end = self.index[0] + offset
 
         # Tick-like, e.g. 3 weeks

pandas/tests/generic/test_generic.py

@@ -912,7 +912,7 @@ def test_equals(self):
 
     def test_pipe(self):
         df = DataFrame({"A": [1, 2, 3]})
-        f = lambda x, y: x ** y
+        def f(x, y): return x ** y
         result = df.pipe(f, 2)
         expected = DataFrame({"A": [1, 4, 9]})
         tm.assert_frame_equal(result, expected)
@@ -922,7 +922,7 @@ def test_pipe(self):
 
     def test_pipe_tuple(self):
         df = DataFrame({"A": [1, 2, 3]})
-        f = lambda x, y: y
+        def f(x, y): return y
         result = df.pipe((f, "y"), 0)
         tm.assert_frame_equal(result, df)
 
@@ -931,7 +931,7 @@ def test_pipe_tuple(self):
 
     def test_pipe_tuple_error(self):
         df = DataFrame({"A": [1, 2, 3]})
-        f = lambda x, y: y
+        def f(x, y): return y
         with pytest.raises(ValueError):
             df.pipe((f, "y"), x=1, y=0)
 
@@ -970,3 +970,10 @@ def test_deprecated_get_dtype_counts(self):
         df = DataFrame([1])
         with tm.assert_produces_warning(FutureWarning):
             df.get_dtype_counts()
+
+    # test case where offset and index[0] overlap
+    def test_first_index_series_last_day_of_month(self):
+        series = pd.Series(1, index=pd.bdate_range('2010-03-31', periods=100))
+        res = series.first('1M')
+        assert("2010-03-31" in res)
+        assert("2010-04-30" not in res)