Fix: avoid object dtype inference warning in to_datetime

Author: jorisvandenbossche

pandas/core/tools/datetimes.py

@@ -16,6 +16,8 @@
 
 import numpy as np
 
+from pandas._config import using_string_dtype
+
 from pandas._libs import (
     lib,
     tslib,
@@ -476,6 +478,9 @@ def _array_strptime_with_fallback(
         unit = np.datetime_data(result.dtype)[0]
         res = Index(result, dtype=f"M8[{unit}, UTC]", name=name)
         return res
+    elif using_string_dtype() and result.dtype == object:
+        if lib.is_string_array(result):
+            return Index(result, dtype="str", name=name)
     return Index(result, dtype=result.dtype, name=name)
 
 

pandas/tests/tools/test_to_datetime.py

@@ -1492,7 +1492,9 @@ def test_datetime_invalid_index(self, values, format):
             warn, match="Could not infer format", raise_on_extra_warnings=False
         ):
             res = to_datetime(values, errors="ignore", format=format)
-        tm.assert_index_equal(res, Index(values, dtype=object))
+        tm.assert_index_equal(
+            res, Index(values, dtype="object" if format is None else "str")
+        )
 
         with tm.assert_produces_warning(
             warn, match="Could not infer format", raise_on_extra_warnings=False
@@ -3713,7 +3715,7 @@ def test_to_datetime_mixed_not_necessarily_iso8601_raise():
     ("errors", "expected"),
     [
         ("coerce", DatetimeIndex(["2020-01-01 00:00:00", NaT])),
-        ("ignore", Index(["2020-01-01", "01-01-2000"], dtype=object)),
+        ("ignore", Index(["2020-01-01", "01-01-2000"], dtype="str")),
     ],
 )
 def test_to_datetime_mixed_not_necessarily_iso8601_coerce(errors, expected):