fixup

Author: MarcoGorelli

pandas/_libs/tslib.pyx

@@ -421,6 +421,20 @@ def array_with_unit_to_datetime(
 
     return oresult, tz
 
+def first_non_null(values: ndarray):
+    """Find first non-null value, return None if there isn't one."""
+    cdef:
+        Py_ssize_t n = len(values)
+        Py_ssize_t i
+    for i in range(n):
+        val = values[i]
+        if checknull_with_nat_and_na(val):
+            continue
+        if isinstance(val, str) and (len(val) == 0 or val in nat_strings):
+            continue
+        return val
+    else:
+        return None
 
 @cython.wraparound(False)
 @cython.boundscheck(False)

pandas/core/tools/datetimes.py

@@ -129,19 +129,16 @@ class FulldatetimeDict(YearMonthDayDict, total=False):
 
 def _guess_datetime_format_for_array(arr, dayfirst: bool | None = False) -> str | None:
     # Try to guess the format based on the first non-NaN element, return None if can't
-    if len(non_nan_elements := notna(arr).nonzero()[0]):
-        if type(first_non_nan_element := arr[non_nan_elements[0]]) is str:
-            # GH#32264 np.str_ object
-            guessed_format = guess_datetime_format(
-                first_non_nan_element, dayfirst=dayfirst
-            )
-            if guessed_format is not None:
-                return guessed_format
-            warnings.warn(
-                "Could not infer format - "
-                "to ensure consistent parsing, specify a format.",
-                stacklevel=find_stack_level(),
-            )
+    if type(first_non_nan_element := tslib.first_non_null(arr)) is str:
+        # GH#32264 np.str_ object
+        guessed_format = guess_datetime_format(first_non_nan_element, dayfirst=dayfirst)
+        if guessed_format is not None:
+            return guessed_format
+        warnings.warn(
+            "Could not infer format - "
+            "to ensure consistent parsing, specify a format.",
+            stacklevel=find_stack_level(),
+        )
     return None
 
 

pandas/tests/tools/test_to_datetime.py

@@ -2100,7 +2100,7 @@ def test_to_datetime_dta_tz(self, klass):
 
 
 class TestGuessDatetimeFormat:
-    @td.skip_if_not_us_locale
+    # @td.skip_if_not_us_locale
     @pytest.mark.parametrize(
         "test_array",
         [
@@ -2110,6 +2110,8 @@ class TestGuessDatetimeFormat:
                 "2011-12-30 00:00:00.000000",
             ],
             [np.nan, np.nan, "2011-12-30 00:00:00.000000"],
+            ["", "2011-12-30 00:00:00.000000"],
+            ["NaT", "2011-12-30 00:00:00.000000"],
             ["2011-12-30 00:00:00.000000", "random_string"],
         ],
     )