fix for Python 3.7: re._pattern_type no longer exist (#849)

* fix for Python 3.7: re._pattern_type no longer exist

* re._pattern_type was removed in Python 3.7. re.Pattern can be used
instead.
* Try re.Pattern first, and fall back to re._pattern_type otherwise.

* Update try..except block to if..else for performance

* Also add check in test_series.py to check for supported behavior in
  regex.

modin/pandas/series.py

@@ -15,6 +15,13 @@
 from .utils import _inherit_docstrings
 from .utils import from_pandas, to_pandas
 
+if sys.version_info[0] == 3 and sys.version_info[1] >= 7:
+    # Python >= 3.7
+    from re import Pattern as _pattern_type
+else:
+    # Python <= 3.6
+    from re import _pattern_type
+
 
 @_inherit_docstrings(pandas.Series, excluded=[pandas.Series, pandas.Series.__init__])
 class Series(BasePandasDataset):
@@ -1434,9 +1441,7 @@ def slice_replace(self, start=None, stop=None, repl=None):
         )
 
     def count(self, pat, flags=0, **kwargs):
-        import re
-
-        if not isinstance(pat, (str, re._pattern_type)):
+        if not isinstance(pat, (str, _pattern_type)):
             raise TypeError("first argument must be string or compiled pattern")
         return Series(
             query_compiler=self._query_compiler.str_count(pat, flags=flags, **kwargs)
@@ -1449,18 +1454,14 @@ def endswith(self, pat, na=np.NaN):
         return Series(query_compiler=self._query_compiler.str_endswith(pat, na=na))
 
     def findall(self, pat, flags=0, **kwargs):
-        import re
-
-        if not isinstance(pat, (str, re._pattern_type)):
+        if not isinstance(pat, (str, _pattern_type)):
             raise TypeError("first argument must be string or compiled pattern")
         return Series(
             query_compiler=self._query_compiler.str_findall(pat, flags=flags, **kwargs)
         )
 
     def match(self, pat, case=True, flags=0, na=np.NaN):
-        import re
-
-        if not isinstance(pat, (str, re._pattern_type)):
+        if not isinstance(pat, (str, _pattern_type)):
             raise TypeError("first argument must be string or compiled pattern")
         return Series(
             query_compiler=self._query_compiler.str_match(pat, flags=flags, na=na)

modin/pandas/test/test_series.py

@@ -5,6 +5,7 @@
 import matplotlib
 import modin.pandas as pd
 from numpy.testing import assert_array_equal
+import sys
 
 from modin.pandas.utils import to_pandas
 from .utils import (
@@ -2717,6 +2718,10 @@ def test_str_cat():
 @pytest.mark.parametrize("n", int_arg_values, ids=int_arg_keys)
 @pytest.mark.parametrize("expand", bool_arg_values, ids=bool_arg_keys)
 def test_str_split(data, pat, n, expand):
+    # Empty pattern not supported on Python 3.7+
+    if sys.version_info[0] == 3 and sys.version_info[1] >= 7 and pat == "":
+        return
+
     modin_series, pandas_series = create_test_series(data)
 
     if n >= -1: