deprecate nonkeywordargs in interpolate

doc/source/whatsnew/v1.3.0.rst

@@ -647,6 +647,7 @@ Deprecations
 - Deprecated setting :attr:`Categorical._codes`, create a new :class:`Categorical` with the desired codes instead (:issue:`40606`)
 - Deprecated behavior of :meth:`DatetimeIndex.union` with mixed timezones; in a future version both will be cast to UTC instead of object dtype (:issue:`39328`)
 - Deprecated using ``usecols`` with out of bounds indices for ``read_csv`` with ``engine="c"`` (:issue:`25623`)
+- Deprecated passing arguments as positional in :meth:`DataFrame.interpolate` and :meth:`Series.interpolate` (:issue:`41485`)
 
 .. ---------------------------------------------------------------------------
 

pandas/core/generic.py

@@ -61,6 +61,7 @@
     InvalidIndexError,
 )
 from pandas.util._decorators import (
+    deprecate_nonkeyword_arguments,
     doc,
     rewrite_axis_style_signature,
 )
@@ -6696,6 +6697,7 @@ def replace(
         else:
             return result.__finalize__(self, method="replace")
 
+    @deprecate_nonkeyword_arguments(version="2.0", allowed_args=["self"])
     @final
     def interpolate(
         self: FrameOrSeries,

pandas/tests/frame/methods/test_interpolate.py

@@ -342,3 +342,13 @@ def test_interp_fillna_methods(self, axis, method):
         expected = df.fillna(axis=axis, method=method)
         result = df.interpolate(method=method, axis=axis)
         tm.assert_frame_equal(result, expected)
+
+    def test_interpolate_pos_args_deprecation(self):
+        # https://github.com/pandas-dev/pandas/issues/41485
+        df = DataFrame({"a": [1, 2, 3]})
+        msg = (
+            r"Starting with Pandas version 2\.0 all arguments of interpolate except "
+            r"for the argument 'self' will be keyword-only"
+        )
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            df.interpolate(0)

pandas/tests/resample/test_datetime_index.py

@@ -1026,12 +1026,14 @@ def test_resample_dtype_coercion():
     df = {"a": [1, 3, 1, 4]}
     df = DataFrame(df, index=date_range("2017-01-01", "2017-01-04"))
 
-    expected = df.astype("float64").resample("H").mean()["a"].interpolate("cubic")
+    expected = (
+        df.astype("float64").resample("H").mean()["a"].interpolate(method="cubic")
+    )
 
-    result = df.resample("H")["a"].mean().interpolate("cubic")
+    result = df.resample("H")["a"].mean().interpolate(method="cubic")
     tm.assert_series_equal(result, expected)
 
-    result = df.resample("H").mean()["a"].interpolate("cubic")
+    result = df.resample("H").mean()["a"].interpolate(method="cubic")
     tm.assert_series_equal(result, expected)
 
 

pandas/tests/series/methods/test_interpolate.py

@@ -811,3 +811,13 @@ 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_pos_args_deprecation(self):
+        # https://github.com/pandas-dev/pandas/issues/41485
+        ser = Series([1, 2, 3])
+        msg = (
+            r"Starting with Pandas version 2\.0 all arguments of interpolate except "
+            r"for the argument 'self' will be keyword-only"
+        )
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            ser.interpolate(0)