Adding tests for pandas-dev#821

Author: Laurent Mutricy

tests/test_series.py

@@ -32,6 +32,7 @@
 from typing_extensions import (
     Self,
     TypeAlias,
+    assert_never,
     assert_type,
 )
 import xarray as xr
@@ -3069,3 +3070,48 @@ def first_arg_not_series(argument_1: int, ser: pd.Series) -> pd.Series:
             ),
             1,
         )
+
+
+def test_diff() -> None:
+    s = pd.Series([1, 1, 2, 3, 5, 8])
+    # int -> float
+    check(assert_type(s.diff(), "pd.Series[float]"), pd.Series, float)
+    # unint -> float
+    check(assert_type(s.astype(np.uint32).diff(), "pd.Series[float]"), pd.Series, float)
+    # float -> float
+    check(assert_type(s.astype(float).diff(), "pd.Series[float]"), pd.Series, float)
+    # timestamp -> timedelta
+    times = pd.Series([pd.Timestamp(0), pd.Timestamp(1)])
+    check(assert_type(times.diff(), "TimedeltaSeries"), pd.Series, pd.Timedelta)
+    # timedelta -> timedelta64
+    check(
+        assert_type(
+            pd.Series([pd.Timedelta(0), pd.Timedelta(1)]).diff(), "TimedeltaSeries"
+        ),
+        pd.Series,
+        pd.Timedelta,
+    )
+    # period -> object
+    check(
+        assert_type(
+            pd.Series(
+                [pd.Period("2012", freq="D"), pd.Period("2012-1-1", freq="D")]
+            ).diff(),
+            "pd.Series[object]",
+        ),
+        pd.Series,
+        object,
+    )
+    # bool -> object
+    check(
+        assert_type(
+            pd.Series([True, True, False, False, True]).diff(), "pd.Series[object]"
+        ),
+        pd.Series,
+        object,
+    )
+    # object -> object
+    check(assert_type(s.astype(object).diff(), "pd.Series[object]"), pd.Series, object)
+    # interval -> TypeError: IntervalArray has no 'diff' method. Convert to a suitable dtype prior to calling 'diff'.
+    if TYPE_CHECKING_INVALID_USAGE:
+        assert_never(pd.Series([pd.Interval(0, 2), pd.Interval(1, 4)]).diff())