DEPR: Series.first() and DataFrame.first()

doc/source/whatsnew/v2.1.0.rst

@@ -269,7 +269,7 @@ Deprecations
 - Deprecated behavior of :func:`assert_series_equal` and :func:`assert_frame_equal` considering NA-like values (e.g. ``NaN`` vs ``None`` as equivalent) (:issue:`52081`)
 - Deprecated constructing :class:`SparseArray` from scalar data, pass a sequence instead (:issue:`53039`)
 - Deprecated positional indexing on :class:`Series` with :meth:`Series.__getitem__` and :meth:`Series.__setitem__`, in a future version ``ser[item]`` will *always* interpret ``item`` as a label, not a position (:issue:`50617`)
--
+- Deprecated :meth:`Series.first` and :meth:`DataFrame.fist` (:issue:`45908`), create a mask and filter using ``.loc`` instead
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_210.performance:

pandas/core/generic.py

@@ -9162,6 +9162,12 @@ def first(self, offset) -> Self:
         3 days observed in the dataset, and therefore data for 2018-04-13 was
         not returned.
         """
+        warnings.warn(
+            "first is deprecated and will be removed in a future version"
+            "please create a mask and filter using `.loc` instead",
+            FutureWarning,
+            stacklevel=find_stack_level(),
+        )
         if not isinstance(self.index, DatetimeIndex):
             raise TypeError("'first' only supports a DatetimeIndex index")
 

pandas/tests/frame/methods/test_first_and_last.py

@@ -10,37 +10,48 @@
 )
 import pandas._testing as tm
 
+deprecated_msg = "first is deprecated"
+
 
 class TestFirst:
     def test_first_subset(self, frame_or_series):
         ts = tm.makeTimeDataFrame(freq="12h")
         ts = tm.get_obj(ts, frame_or_series)
-        result = ts.first("10d")
-        assert len(result) == 20
+        with tm.assert_produces_warning(FutureWarning, match=deprecated_msg):
+            result = ts.first("10d")
+            assert len(result) == 20
 
         ts = tm.makeTimeDataFrame(freq="D")
         ts = tm.get_obj(ts, frame_or_series)
-        result = ts.first("10d")
-        assert len(result) == 10
+        with tm.assert_produces_warning(FutureWarning, match=deprecated_msg):
+            result = ts.first("10d")
+            assert len(result) == 10
 
-        result = ts.first("3M")
-        expected = ts[:"3/31/2000"]
-        tm.assert_equal(result, expected)
+        with tm.assert_produces_warning(FutureWarning, match=deprecated_msg):
+            result = ts.first("3M")
+            expected = ts[:"3/31/2000"]
+            tm.assert_equal(result, expected)
 
-        result = ts.first("21D")
-        expected = ts[:21]
-        tm.assert_equal(result, expected)
+        with tm.assert_produces_warning(FutureWarning, match=deprecated_msg):
+            result = ts.first("21D")
+            expected = ts[:21]
+            tm.assert_equal(result, expected)
 
-        result = ts[:0].first("3M")
-        tm.assert_equal(result, ts[:0])
+        with tm.assert_produces_warning(FutureWarning, match=deprecated_msg):
+            result = ts[:0].first("3M")
+            tm.assert_equal(result, ts[:0])
 
     def test_first_last_raises(self, frame_or_series):
         # GH#20725
         obj = DataFrame([[1, 2, 3], [4, 5, 6]])
         obj = tm.get_obj(obj, frame_or_series)
 
         msg = "'first' only supports a DatetimeIndex index"
-        with pytest.raises(TypeError, match=msg):  # index is not a DatetimeIndex
+        with tm.assert_produces_warning(
+            FutureWarning, match=deprecated_msg
+        ), pytest.raises(
+            TypeError, match=msg
+        ):  # index is not a DatetimeIndex
             obj.first("1D")
 
         msg = "'last' only supports a DatetimeIndex index"
@@ -73,7 +84,8 @@ def test_last_subset(self, frame_or_series):
     def test_first_with_first_day_last_of_month(self, frame_or_series, start, periods):
         # GH#29623
         x = frame_or_series([1] * 100, index=bdate_range(start, periods=100))
-        result = x.first("1M")
+        with tm.assert_produces_warning(FutureWarning, match=deprecated_msg):
+            result = x.first("1M")
         expected = frame_or_series(
             [1] * periods, index=bdate_range(start, periods=periods)
         )
@@ -82,16 +94,20 @@ def test_first_with_first_day_last_of_month(self, frame_or_series, start, period
     def test_first_with_first_day_end_of_frq_n_greater_one(self, frame_or_series):
         # GH#29623
         x = frame_or_series([1] * 100, index=bdate_range("2010-03-31", periods=100))
-        result = x.first("2M")
+        with tm.assert_produces_warning(FutureWarning, match=deprecated_msg):
+            result = x.first("2M")
         expected = frame_or_series(
             [1] * 23, index=bdate_range("2010-03-31", "2010-04-30")
         )
         tm.assert_equal(result, expected)
 
-    @pytest.mark.parametrize("func", ["first", "last"])
-    def test_empty_not_input(self, func):
+    def test_empty_not_input(self):
         # GH#51032
         df = DataFrame(index=pd.DatetimeIndex([]))
-        result = getattr(df, func)(offset=1)
+        result = df.last(offset=1)
+
+        with tm.assert_produces_warning(FutureWarning, match=deprecated_msg):
+            result = df.first(offset=1)
+
         tm.assert_frame_equal(df, result)
         assert df is not result