DEPR: silent string casting in binary Timedelta operations

doc/source/whatsnew/v3.0.0.rst

@@ -742,6 +742,7 @@ Other Deprecations
 - Deprecated option "future.no_silent_downcasting", as it is no longer used. In a future version accessing this option will raise (:issue:`59502`)
 - Deprecated passing non-Index types to :meth:`Index.join`; explicitly convert to Index first (:issue:`62897`)
 - Deprecated silent casting of non-datetime 'other' to datetime in :meth:`Series.combine_first` (:issue:`62931`)
+- Deprecated silently casting strings to :class:`Timedelta` in binary operations with :class:`Timedelta` (:issue:`59653`)
 - Deprecated slicing on a :class:`Series` or :class:`DataFrame` with a :class:`DatetimeIndex` using a ``datetime.date`` object, explicitly cast to :class:`Timestamp` instead (:issue:`35830`)
 - Deprecated support for the Dataframe Interchange Protocol (:issue:`56732`)
 - Deprecated the 'inplace' keyword from :meth:`Resampler.interpolate`, as passing ``True`` raises ``AttributeError`` (:issue:`58690`)

pandas/_libs/tslibs/timedeltas.pyx

@@ -798,7 +798,7 @@ def _binary_op_method_timedeltalike(op, name):
             return NotImplemented
 
         try:
-            other = Timedelta(other)
+            other = _wrapped_to_timedelta(other)
         except ValueError:
             # failed to parse as timedelta
             return NotImplemented
@@ -2341,7 +2341,7 @@ class Timedelta(_Timedelta):
     def __truediv__(self, other):
         if _should_cast_to_timedelta(other):
             # We interpret NaT as timedelta64("NaT")
-            other = Timedelta(other)
+            other = _wrapped_to_timedelta(other)
             if other is NaT:
                 return np.nan
             if other._creso != self._creso:
@@ -2374,7 +2374,7 @@ class Timedelta(_Timedelta):
     def __rtruediv__(self, other):
         if _should_cast_to_timedelta(other):
             # We interpret NaT as timedelta64("NaT")
-            other = Timedelta(other)
+            other = _wrapped_to_timedelta(other)
             if other is NaT:
                 return np.nan
             if self._creso != other._creso:
@@ -2402,7 +2402,7 @@ class Timedelta(_Timedelta):
         # just defer
         if _should_cast_to_timedelta(other):
             # We interpret NaT as timedelta64("NaT")
-            other = Timedelta(other)
+            other = _wrapped_to_timedelta(other)
             if other is NaT:
                 return np.nan
             if self._creso != other._creso:
@@ -2457,7 +2457,7 @@ class Timedelta(_Timedelta):
         # just defer
         if _should_cast_to_timedelta(other):
             # We interpret NaT as timedelta64("NaT")
-            other = Timedelta(other)
+            other = _wrapped_to_timedelta(other)
             if other is NaT:
                 return np.nan
             if self._creso != other._creso:
@@ -2525,6 +2525,7 @@ def truediv_object_array(ndarray left, ndarray right):
         if cnp.get_timedelta64_value(td64) == NPY_NAT:
             # td here should be interpreted as a td64 NaT
             if _should_cast_to_timedelta(obj):
+                _wrapped_to_timedelta(obj)  # deprecate if allowing string
                 res_value = np.nan
             else:
                 # if its a number then let numpy handle division, otherwise
@@ -2554,6 +2555,7 @@ def floordiv_object_array(ndarray left, ndarray right):
         if cnp.get_timedelta64_value(td64) == NPY_NAT:
             # td here should be interpreted as a td64 NaT
             if _should_cast_to_timedelta(obj):
+                _wrapped_to_timedelta(obj)  # deprecate allowing string
                 res_value = np.nan
             else:
                 # if its a number then let numpy handle division, otherwise
@@ -2585,6 +2587,23 @@ cdef bint is_any_td_scalar(object obj):
     )
 
 
+cdef inline _wrapped_to_timedelta(object other):
+    # Helper for deprecating cases where we cast str to Timedelta
+    td = Timedelta(other)
+    if isinstance(other, str):
+        from pandas.errors import Pandas4Warning
+        warnings.warn(
+            # GH#59653
+            "Scalar operations between Timedelta and string are "
+            "deprecated and will raise in a future version. "
+            "Explicitly cast to Timedelta first.",
+            Pandas4Warning,
+            stacklevel=find_stack_level(),
+        )
+        # When this is enforced, remove str from _should_cast_to_timedelta
+    return td
+
+
 cdef bint _should_cast_to_timedelta(object obj):
     """
     Should we treat this object as a Timedelta for the purpose of a binary op

pandas/tests/scalar/timedelta/test_arithmetic.py

@@ -11,7 +11,10 @@
 import numpy as np
 import pytest
 
-from pandas.errors import OutOfBoundsTimedelta
+from pandas.errors import (
+    OutOfBoundsTimedelta,
+    Pandas4Warning,
+)
 
 import pandas as pd
 from pandas import (
@@ -1182,3 +1185,45 @@ def test_ops_error_str():
 
         assert not left == right
         assert left != right
+
+
+@pytest.mark.parametrize("box", [True, False])
+def test_ops_str_deprecated(box):
+    # GH#59653
+    td = Timedelta("1 day")
+    item = "1"
+    if box:
+        item = np.array([item], dtype=object)
+
+    msg = "Scalar operations between Timedelta and string are deprecated"
+    with tm.assert_produces_warning(Pandas4Warning, match=msg):
+        td + item
+    with tm.assert_produces_warning(Pandas4Warning, match=msg):
+        item + td
+    with tm.assert_produces_warning(Pandas4Warning, match=msg):
+        td - item
+    with tm.assert_produces_warning(Pandas4Warning, match=msg):
+        item - td
+    with tm.assert_produces_warning(Pandas4Warning, match=msg):
+        item / td
+    if not box:
+        with tm.assert_produces_warning(Pandas4Warning, match=msg):
+            td / item
+        with tm.assert_produces_warning(Pandas4Warning, match=msg):
+            item // td
+        with tm.assert_produces_warning(Pandas4Warning, match=msg):
+            td // item
+    else:
+        msg = "|".join(
+            [
+                "ufunc 'divide' cannot use operands",
+                "Invalid dtype object for __floordiv__",
+                r"unsupported operand type\(s\) for /: 'int' and 'str'",
+            ]
+        )
+        with pytest.raises(TypeError, match=msg):
+            td / item
+        with pytest.raises(TypeError, match=msg):
+            item // td
+        with pytest.raises(TypeError, match=msg):
+            td // item