BUG: to_datetime(None) inconsistent with other conversions (#62249)

Author: jbrockmendel

doc/source/whatsnew/v3.0.0.rst

@@ -921,6 +921,7 @@ Conversion
 - Bug in :meth:`Series.astype` might modify read-only array inplace when casting to a string dtype (:issue:`57212`)
 - Bug in :meth:`Series.convert_dtypes` and :meth:`DataFrame.convert_dtypes` removing timezone information for objects with :class:`ArrowDtype` (:issue:`60237`)
 - Bug in :meth:`Series.reindex` not maintaining ``float32`` type when a ``reindex`` introduces a missing value (:issue:`45857`)
+- Bug in :meth:`to_datetime` and :meth:`to_timedelta` with input ``None`` returning ``None`` instead of ``NaT``, inconsistent with other conversion methods (:issue:`23055`)
 
 Strings
 ^^^^^^^

pandas/core/generic.py

@@ -10519,8 +10519,12 @@ def truncate(
         if ax._is_all_dates:
             from pandas.core.tools.datetimes import to_datetime
 
-            before = to_datetime(before)
-            after = to_datetime(after)
+            if before is not None:
+                # Avoid converting to NaT
+                before = to_datetime(before)
+            if after is not None:
+                # Avoid converting to NaT
+                after = to_datetime(after)
 
         if before is not None and after is not None and before > after:
             raise ValueError(f"Truncate: {after} must be after {before}")

pandas/core/tools/datetimes.py

@@ -21,6 +21,7 @@
     tslib,
 )
 from pandas._libs.tslibs import (
+    NaT,
     OutOfBoundsDatetime,
     Timedelta,
     Timestamp,
@@ -676,7 +677,7 @@ def to_datetime(
     unit: str | None = None,
     origin: str = "unix",
     cache: bool = True,
-) -> DatetimeIndex | Series | DatetimeScalar | NaTType | None:
+) -> DatetimeIndex | Series | DatetimeScalar | NaTType:
     """
     Convert argument to datetime.
 
@@ -989,7 +990,7 @@ def to_datetime(
     if exact is not lib.no_default and format in {"mixed", "ISO8601"}:
         raise ValueError("Cannot use 'exact' when 'format' is 'mixed' or 'ISO8601'")
     if arg is None:
-        return None
+        return NaT
 
     if origin != "unix":
         arg = _adjust_to_origin(arg, origin, unit)

pandas/core/tools/timedeltas.py

@@ -182,7 +182,7 @@ def to_timedelta(
         raise ValueError("errors must be one of 'raise', or 'coerce'.")
 
     if arg is None:
-        return arg
+        return NaT
     elif isinstance(arg, ABCSeries):
         values = _convert_listlike(arg._values, unit=unit, errors=errors)
         return arg._constructor(values, index=arg.index, name=arg.name)

pandas/tests/tools/test_to_datetime.py

@@ -531,6 +531,10 @@ def test_to_datetime_parse_timezone_keeps_name(self):
 
 
 class TestToDatetime:
+    def test_to_datetime_none(self):
+        # GH#23055
+        assert to_datetime(None) is NaT
+
     @pytest.mark.filterwarnings("ignore:Could not infer format")
     def test_to_datetime_overflow(self):
         # we should get an OutOfBoundsDatetime, NOT OverflowError

pandas/tests/tools/test_to_timedelta.py

@@ -27,6 +27,10 @@
 
 
 class TestTimedeltas:
+    def test_to_timedelta_none(self):
+        # GH#23055
+        assert to_timedelta(None) is pd.NaT
+
     def test_to_timedelta_dt64_raises(self):
         # Passing datetime64-dtype data to TimedeltaIndex is no longer
         #  supported GH#29794