BUG: fix datetimes.should_cache() error for deque (GH 29403) (pandas-dev#29846)

Author: nathalier

doc/source/whatsnew/v1.0.0.rst

@@ -701,6 +701,7 @@ Datetimelike
 - Bug in :meth:`DatetimeIndex.strftime` and :meth:`Series.dt.strftime` where ``NaT`` was converted to the string ``'NaT'`` instead of ``np.nan`` (:issue:`29578`)
 - Bug in :attr:`Timestamp.resolution` being a property instead of a class attribute (:issue:`29910`)
 - Bug in :func:`pandas.to_datetime` when called with ``None`` raising ``TypeError`` instead of returning ``NaT`` (:issue:`30011`)
+- Bug in :func:`pandas.to_datetime` failing for `deques` when using ``cache=True`` (the default) (:issue:`29403`)
 
 Timedelta
 ^^^^^^^^^

pandas/core/tools/datetimes.py

@@ -1,6 +1,7 @@
 from collections import abc
 from datetime import datetime, time
 from functools import partial
+from itertools import islice
 from typing import Optional, TypeVar, Union
 
 import numpy as np
@@ -111,7 +112,7 @@ def should_cache(
 
     assert 0 < unique_share < 1, "unique_share must be in next bounds: (0; 1)"
 
-    unique_elements = unique(arg[:check_count])
+    unique_elements = set(islice(arg, check_count))
     if len(unique_elements) > check_count * unique_share:
         do_caching = False
     return do_caching

pandas/tests/indexes/datetimes/test_tools.py

@@ -1,6 +1,7 @@
 """ test to_datetime """
 
 import calendar
+from collections import deque
 from datetime import datetime, time
 import locale
 
@@ -861,7 +862,7 @@ def test_datetime_invalid_index(self, values, format, infer):
 
     @pytest.mark.parametrize("utc", [True, None])
     @pytest.mark.parametrize("format", ["%Y%m%d %H:%M:%S", None])
-    @pytest.mark.parametrize("constructor", [list, tuple, np.array, pd.Index])
+    @pytest.mark.parametrize("constructor", [list, tuple, np.array, pd.Index, deque])
     def test_to_datetime_cache(self, utc, format, constructor):
         date = "20130101 00:00:00"
         test_dates = [date] * 10 ** 5
@@ -872,6 +873,24 @@ def test_to_datetime_cache(self, utc, format, constructor):
 
         tm.assert_index_equal(result, expected)
 
+    @pytest.mark.parametrize(
+        "listlike",
+        [
+            (deque([pd.Timestamp("2010-06-02 09:30:00")] * 51)),
+            ([pd.Timestamp("2010-06-02 09:30:00")] * 51),
+            (tuple([pd.Timestamp("2010-06-02 09:30:00")] * 51)),
+        ],
+    )
+    def test_no_slicing_errors_in_should_cache(self, listlike):
+        # GH 29403
+        assert tools.should_cache(listlike) is True
+
+    def test_to_datetime_from_deque(self):
+        # GH 29403
+        result = pd.to_datetime(deque([pd.Timestamp("2010-06-02 09:30:00")] * 51))
+        expected = pd.to_datetime([pd.Timestamp("2010-06-02 09:30:00")] * 51)
+        tm.assert_index_equal(result, expected)
+
     @pytest.mark.parametrize("utc", [True, None])
     @pytest.mark.parametrize("format", ["%Y%m%d %H:%M:%S", None])
     def test_to_datetime_cache_series(self, utc, format):