ENH: infer Timestamp unit in non-iso paths

pandas/_libs/tslibs/conversion.pyx

@@ -50,29 +50,27 @@ from pandas._libs.tslibs.np_datetime cimport (
 
 from pandas._libs.tslibs.np_datetime import OutOfBoundsDatetime
 
-from pandas._libs.tslibs.timezones cimport (
-    get_utcoffset,
-    is_utc,
-    maybe_get_tz,
-)
-from pandas._libs.tslibs.util cimport (
-    is_datetime64_object,
-    is_float_object,
-    is_integer_object,
-)
-
-from pandas._libs.tslibs.parsing import parse_datetime_string
-
 from pandas._libs.tslibs.nattype cimport (
     NPY_NAT,
     c_NaT as NaT,
     c_nat_strings as nat_strings,
 )
+from pandas._libs.tslibs.parsing cimport parse_datetime_string
 from pandas._libs.tslibs.timestamps cimport _Timestamp
+from pandas._libs.tslibs.timezones cimport (
+    get_utcoffset,
+    is_utc,
+    maybe_get_tz,
+)
 from pandas._libs.tslibs.tzconversion cimport (
     Localizer,
     tz_localize_to_utc_single,
 )
+from pandas._libs.tslibs.util cimport (
+    is_datetime64_object,
+    is_float_object,
+    is_integer_object,
+)
 
 # ----------------------------------------------------------------------
 # Constants
@@ -552,8 +550,10 @@ cdef _TSObject convert_str_to_tsobject(str ts, tzinfo tz, str unit,
                 return obj
 
         dt = parse_datetime_string(
-            ts, dayfirst=dayfirst, yearfirst=yearfirst
+            ts, dayfirst=dayfirst, yearfirst=yearfirst, out_bestunit=&out_bestunit
         )
+        reso = get_supported_reso(out_bestunit)
+        return convert_datetime_to_tsobject(dt, tz, nanos=0, reso=reso)
 
     return convert_datetime_to_tsobject(dt, tz)
 

pandas/_libs/tslibs/parsing.pxd

@@ -1,3 +1,14 @@
+from cpython.datetime cimport datetime
+
+from pandas._libs.tslibs.np_datetime cimport NPY_DATETIMEUNIT
+
 
 cpdef str get_rule_month(str source)
 cpdef quarter_to_myear(int year, int quarter, str freq)
+
+cdef datetime parse_datetime_string(
+    str date_string,
+    bint dayfirst,
+    bint yearfirst,
+    NPY_DATETIMEUNIT* out_bestunit
+)

pandas/_libs/tslibs/parsing.pyi

@@ -6,7 +6,7 @@ from pandas._typing import npt
 
 class DateParseError(ValueError): ...
 
-def parse_datetime_string(
+def py_parse_datetime_string(
     date_string: str,
     dayfirst: bool = ...,
     yearfirst: bool = ...,

pandas/_libs/tslibs/parsing.pyx

@@ -264,14 +264,26 @@ cdef bint _does_string_look_like_time(str parse_string):
     return 0 <= hour <= 23 and 0 <= minute <= 59
 
 
-def parse_datetime_string(
+def py_parse_datetime_string(
+    str date_string, bint dayfirst=False, bint yearfirst=False
+):
+    # Python-accessible version for testing (we can't just make
+    #  parse_datetime_string cpdef bc it has a pointer argument)
+    cdef:
+        NPY_DATETIMEUNIT out_bestunit
+
+    return parse_datetime_string(date_string, dayfirst, yearfirst, &out_bestunit)
+
+
+cdef datetime parse_datetime_string(
     # NB: This will break with np.str_ (GH#32264) even though
     #  isinstance(npstrobj, str) evaluates to True, so caller must ensure
     #  the argument is *exactly* 'str'
     str date_string,
-    bint dayfirst=False,
-    bint yearfirst=False,
-) -> datetime:
+    bint dayfirst,
+    bint yearfirst,
+    NPY_DATETIMEUNIT* out_bestunit
+):
     """
     Parse datetime string, only returns datetime.
     Also cares special handling matching time patterns.
@@ -287,7 +299,6 @@ def parse_datetime_string(
 
     cdef:
         datetime dt
-        NPY_DATETIMEUNIT out_bestunit
         bint is_quarter = 0
 
     if not _does_string_look_like_datetime(date_string):
@@ -299,13 +310,13 @@ def parse_datetime_string(
                       yearfirst=yearfirst)
         return dt
 
-    dt = _parse_delimited_date(date_string, dayfirst, &out_bestunit)
+    dt = _parse_delimited_date(date_string, dayfirst, out_bestunit)
     if dt is not None:
         return dt
 
     try:
         dt = _parse_dateabbr_string(
-            date_string, _DEFAULT_DATETIME, None, &out_bestunit, &is_quarter
+            date_string, _DEFAULT_DATETIME, None, out_bestunit, &is_quarter
         )
         return dt
     except DateParseError:
@@ -315,7 +326,7 @@ def parse_datetime_string(
 
     dt = dateutil_parse(date_string, default=_DEFAULT_DATETIME,
                         dayfirst=dayfirst, yearfirst=yearfirst,
-                        ignoretz=False, out_bestunit=&out_bestunit)
+                        ignoretz=False, out_bestunit=out_bestunit)
     return dt
 
 

pandas/tests/io/parser/test_parse_dates.py

@@ -18,7 +18,7 @@
 import pytz
 
 from pandas._libs.tslibs import parsing
-from pandas._libs.tslibs.parsing import parse_datetime_string
+from pandas._libs.tslibs.parsing import py_parse_datetime_string
 from pandas.compat.pyarrow import (
     pa_version_under6p0,
     pa_version_under7p0,
@@ -1760,7 +1760,7 @@ def test_hypothesis_delimited_date(
     date_string = test_datetime.strftime(date_format.replace(" ", delimiter))
 
     except_out_dateutil, result = _helper_hypothesis_delimited_date(
-        parse_datetime_string, date_string, dayfirst=dayfirst
+        py_parse_datetime_string, date_string, dayfirst=dayfirst
     )
     except_in_dateutil, expected = _helper_hypothesis_delimited_date(
         du_parse,

pandas/tests/scalar/timestamp/test_constructors.py

@@ -34,6 +34,24 @@ def test_construct_from_string_invalid_raises(self):
         with pytest.raises(ValueError, match="gives an invalid tzoffset"):
             Timestamp("200622-12-31")
 
+    def test_constructor_str_infer_reso(self):
+        # non-iso8601 path
+
+        # _parse_delimited_date path
+        ts = Timestamp("01/30/2023")
+        assert ts.unit == "s"
+
+        # _parse_dateabbr_string path
+        ts = Timestamp("2015Q1")
+        assert ts.unit == "s"
+
+        # dateutil_parse path
+        ts = Timestamp("2016-01-01 1:30:01 PM")
+        assert ts.unit == "s"
+
+        ts = Timestamp("2016 June 3 15:25:01.345")
+        assert ts.unit == "ms"
+
     def test_constructor_from_iso8601_str_with_offset_reso(self):
         # GH#49737
         ts = Timestamp("2016-01-01 04:05:06-01:00")