BUG: fix datetimes.should_cache() error for deque (GH 29403)

itertools.islice() should be used to get slice of a deque. itertools.islice() also can be used (and is efficient) for other collections. So
unique(arg[:check_count])
was replaced with
set(islice(arg, check_count))

Author: nathalier

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
@@ -118,7 +119,7 @@ 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
 
@@ -838,7 +839,7 @@ 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
@@ -2275,6 +2276,26 @@ def test_should_cache(listlike, do_caching):
     )
 
 
+@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(listlike):
+    # GH 29403
+    assert tools.should_cache(listlike) is True
+
+
+def test_to_datetime_from_deque():
+    # 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(
     "unique_share,check_count, err_message",
     [