API: timestamp resolution inference: default to microseconds when possible

pandas/_libs/tslib.pyx

@@ -355,7 +355,7 @@ cpdef array_to_datetime(
                 iresult[i] = parse_pydatetime(val, &dts, creso=creso)
 
             elif PyDate_Check(val):
-                item_reso = NPY_DATETIMEUNIT.NPY_FR_s
+                item_reso = NPY_DATETIMEUNIT.NPY_FR_us
                 state.update_creso(item_reso)
                 if infer_reso:
                     creso = state.creso

pandas/_libs/tslibs/conversion.pyx

@@ -33,6 +33,7 @@ from pandas._libs.missing cimport checknull_with_nat_and_na
 from pandas._libs.tslibs.dtypes cimport (
     abbrev_to_npy_unit,
     get_supported_reso,
+    get_supported_reso_for_dts,
     npy_unit_to_attrname,
     periods_per_second,
 )
@@ -507,6 +508,9 @@ cdef _TSObject convert_datetime_to_tsobject(
     if nanos:
         obj.dts.ps = nanos * 1000
 
+    reso = get_supported_reso_for_dts(reso, &obj.dts)
+    obj.creso = reso
+
     try:
         obj.value = npy_datetimestruct_to_datetime(reso, &obj.dts)
     except OverflowError as err:
@@ -622,7 +626,7 @@ cdef _TSObject convert_str_to_tsobject(str ts, tzinfo tz,
                 &out_tzoffset, False
             )
             if not string_to_dts_failed:
-                reso = get_supported_reso(out_bestunit)
+                reso = get_supported_reso_for_dts(out_bestunit, &dts)
                 check_dts_bounds(&dts, reso)
                 obj = _TSObject()
                 obj.dts = dts

pandas/_libs/tslibs/dtypes.pxd

@@ -1,6 +1,9 @@
 from numpy cimport int64_t
 
-from pandas._libs.tslibs.np_datetime cimport NPY_DATETIMEUNIT
+from pandas._libs.tslibs.np_datetime cimport (
+    NPY_DATETIMEUNIT,
+    npy_datetimestruct,
+)
 
 
 cdef str npy_unit_to_abbrev(NPY_DATETIMEUNIT unit)
@@ -9,6 +12,9 @@ cdef NPY_DATETIMEUNIT freq_group_code_to_npy_unit(int freq) noexcept nogil
 cpdef int64_t periods_per_day(NPY_DATETIMEUNIT reso=*) except? -1
 cpdef int64_t periods_per_second(NPY_DATETIMEUNIT reso) except? -1
 cdef NPY_DATETIMEUNIT get_supported_reso(NPY_DATETIMEUNIT reso)
+cdef NPY_DATETIMEUNIT get_supported_reso_for_dts(
+    NPY_DATETIMEUNIT reso, npy_datetimestruct* dts
+)
 cdef bint is_supported_unit(NPY_DATETIMEUNIT reso)
 
 cdef dict c_OFFSET_TO_PERIOD_FREQSTR

pandas/_libs/tslibs/dtypes.pyx

@@ -2,11 +2,21 @@
 # originals
 from enum import Enum
 
+import numpy as np
+
+from cpython.object cimport (
+    Py_GE,
+    Py_LE,
+)
+
 from pandas._libs.tslibs.ccalendar cimport c_MONTH_NUMBERS
 from pandas._libs.tslibs.np_datetime cimport (
     NPY_DATETIMEUNIT,
+    cmp_dtstructs,
     get_conversion_factor,
     import_pandas_datetime,
+    npy_datetimestruct,
+    pandas_datetime_to_datetimestruct,
 )
 
 import_pandas_datetime()
@@ -504,6 +514,36 @@ cdef NPY_DATETIMEUNIT get_supported_reso(NPY_DATETIMEUNIT reso):
     return reso
 
 
+cdef npy_datetimestruct dts_us_min, dts_us_max
+pandas_datetime_to_datetimestruct(
+    np.iinfo(np.int64).min + 1, NPY_DATETIMEUNIT.NPY_FR_us, &dts_us_min
+)
+pandas_datetime_to_datetimestruct(
+    np.iinfo(np.int64).max, NPY_DATETIMEUNIT.NPY_FR_us, &dts_us_max
+)
+
+
+cdef NPY_DATETIMEUNIT get_supported_reso_for_dts(
+    NPY_DATETIMEUNIT reso, npy_datetimestruct* dts
+):
+    # Similar as above, but taking the actual datetime value in account,
+    # defaulting to 'us' if possible.
+    if reso == NPY_DATETIMEUNIT.NPY_FR_GENERIC:
+        return NPY_DATETIMEUNIT.NPY_FR_ns
+    if reso < NPY_DATETIMEUNIT.NPY_FR_us:
+        if (
+            cmp_dtstructs(dts, &dts_us_min, Py_GE)
+            and cmp_dtstructs(dts, &dts_us_max, Py_LE)
+        ):
+            return NPY_DATETIMEUNIT.NPY_FR_us
+        else:
+            # TODO still distinguish between ms or s?
+            return NPY_DATETIMEUNIT.NPY_FR_s
+    elif reso > NPY_DATETIMEUNIT.NPY_FR_ns:
+        return NPY_DATETIMEUNIT.NPY_FR_ns
+    return reso
+
+
 cdef bint is_supported_unit(NPY_DATETIMEUNIT reso):
     return (
         reso == NPY_DATETIMEUNIT.NPY_FR_ns

pandas/_libs/tslibs/strptime.pyx

@@ -53,6 +53,7 @@ from pandas._libs.tslibs.conversion cimport (
 )
 from pandas._libs.tslibs.dtypes cimport (
     get_supported_reso,
+    get_supported_reso_for_dts,
     npy_unit_to_abbrev,
     npy_unit_to_attrname,
 )
@@ -421,7 +422,7 @@ def array_strptime(
                 continue
             elif PyDate_Check(val):
                 state.found_other = True
-                item_reso = NPY_DATETIMEUNIT.NPY_FR_s
+                item_reso = NPY_DATETIMEUNIT.NPY_FR_us
                 state.update_creso(item_reso)
                 if infer_reso:
                     creso = state.creso
@@ -460,7 +461,7 @@ def array_strptime(
             if string_to_dts_succeeded:
                 # No error reported by string_to_dts, pick back up
                 # where we left off
-                item_reso = get_supported_reso(out_bestunit)
+                item_reso = get_supported_reso_for_dts(out_bestunit, &dts)
                 state.update_creso(item_reso)
                 if infer_reso:
                     creso = state.creso
@@ -622,7 +623,7 @@ cdef tzinfo _parse_with_format(
                 f"time data \"{val}\" doesn't match format \"{fmt}\""
             )
 
-    item_reso[0] = NPY_DATETIMEUNIT.NPY_FR_s
+    item_reso[0] = NPY_DATETIMEUNIT.NPY_FR_us
 
     iso_year = -1
     year = 1900
@@ -710,11 +711,7 @@ cdef tzinfo _parse_with_format(
         elif parse_code == 10:
             # e.g. val='10:10:10.100'; fmt='%H:%M:%S.%f'
             s = found_dict["f"]
-            if len(s) <= 3:
-                item_reso[0] = NPY_DATETIMEUNIT.NPY_FR_ms
-            elif len(s) <= 6:
-                item_reso[0] = NPY_DATETIMEUNIT.NPY_FR_us
-            else:
+            if len(s) > 6:
                 item_reso[0] = NPY_FR_ns
             # Pad to always return nanoseconds
             s += "0" * (9 - len(s))