BUG fixing module first() and DateOffset

pandas/core/generic.py

@@ -40,7 +40,9 @@
 from pandas._libs.tslibs import (
     Period,
     Tick,
+    Timedelta,
     Timestamp,
+    offsets,
     to_offset,
 )
 from pandas._typing import (
@@ -9076,9 +9078,20 @@ def first(self, offset) -> Self:
 
         if len(self.index) == 0:
             return self.copy(deep=False)
+        if type(offset) is not offsets.DateOffset:
+            offset = to_offset(offset)
 
-        offset = to_offset(offset)
-        if not isinstance(offset, Tick) and offset.is_on_offset(self.index[0]):
+        if type(offset) is offsets.DateOffset and offset.is_on_offset(self.index[0]):
+            end_date = self.index[0] - Timedelta(1, unit="d") + offset
+
+            return self.loc[:end_date]
+
+        # if not isinstance(offset, Tick) and offset.is_on_offset(self.index[0]):
+        if (
+            not isinstance(offset, Tick)
+            and offset.is_on_offset(self.index[0])
+            and type(offset) is not offsets.DateOffset
+        ):
             # GH#29623 if first value is end of period, remove offset with n = 1
             #  before adding the real offset
             end_date = end = self.index[0] - offset.base + offset

pandas/tests/frame/methods/test_first_and_last.py

@@ -1,13 +1,17 @@
 """
 Note: includes tests for `last`
 """
+import numpy as np
 import pytest
 
 import pandas as pd
 from pandas import (
     DataFrame,
+    Series,
     bdate_range,
 )
+
+# from pandas._libs.tslibs import offsets
 import pandas._testing as tm
 
 
@@ -95,3 +99,39 @@ def test_empty_not_input(self, func):
         result = getattr(df, func)(offset=1)
         tm.assert_frame_equal(df, result)
         assert df is not result
+
+    @pytest.mark.parametrize("start, periods", [("2010-03-31", 1), ("2010-03-30", 2)])
+    def test_first_with_first_day_last_of_m_DO(self, frame_or_series, start, periods):
+        x = frame_or_series([1] * 100, index=bdate_range(start, periods=100))
+        result = x.first(pd.DateOffset(days=periods))
+        expected = frame_or_series(
+            [1] * periods, index=bdate_range(start, periods=periods)
+        )
+        tm.assert_equal(result, expected)
+
+    def test_first_with_first_day_end_of_frq_n_greater_one_DO(self, frame_or_series):
+        x = frame_or_series([1] * 100, index=bdate_range("2010-03-31", periods=100))
+        result = x.first(pd.DateOffset(days=2))
+        expected = frame_or_series(
+            [1] * 2, index=bdate_range("2010-03-31", "2010-04-01")
+        )
+        tm.assert_equal(result, expected)
+
+    def test_first_w_DateOffset(self):
+        # GH#51284
+        i = pd.date_range("2018-04-09", periods=4, freq="2D")
+        x = DataFrame({"A": [1, 2, 3, 4]}, index=i)
+        result = x.first(pd.DateOffset(days=3))
+        expected = DataFrame(
+            {"A": [1, 2]}, index=pd.date_range("2018-04-09", periods=2, freq="2D")
+        )
+        tm.assert_equal(result, expected)
+
+    def test_first_w_DateOffset_other(self):
+        # GH#45908
+        i = pd.date_range("2018-04-09", periods=30, freq="2D")
+        x = DataFrame({"A": Series(np.arange(30), index=i)}, index=i)
+        result = x.first(pd.DateOffset(days=15))
+        i2 = pd.date_range("2018-04-09", periods=8, freq="2D")
+        expected = DataFrame({"A": Series(np.arange(8), index=i2)}, index=i2)
+        tm.assert_equal(result, expected)