API: change default for sep in str.cat (in docstring)

pandas/core/strings.py

@@ -2074,9 +2074,10 @@ def cat(self, others=None, sep=None, na_rep=None, join=None):
 
  If others is None, the method returns the concatenation of all
  strings in the calling Series/Index.
- sep : string or None, default None
- If None, concatenates without any separator.
- na_rep : string or None, default None
+ sep : str, default ''
+ The separator between the different elements/columns. By default
+ the empty string `''` is used.
+ na_rep : str or None, default None
  Representation that is inserted for all missing values:
 
  - If `na_rep` is None, and `others` is None, missing values in the

pandas/tests/test_strings.py

@@ -162,10 +162,11 @@ def test_str_cat_raises_intuitive_error(self, box):
  with tm.assert_raises_regex(ValueError, message):
  s.str.cat(' ')
 
+ @pytest.mark.parametrize('sep', ['', None])
  @pytest.mark.parametrize('dtype_target', ['object', 'category'])
  @pytest.mark.parametrize('dtype_caller', ['object', 'category'])
  @pytest.mark.parametrize('box', [Series, Index])
- def test_str_cat_categorical(self, box, dtype_caller, dtype_target):
+ def test_str_cat_categorical(self, box, dtype_caller, dtype_target, sep):
  s = Index(['a', 'a', 'b', 'a'], dtype=dtype_caller)
  s = s if box == Index else Series(s, index=s)
  t = Index(['b', 'a', 'b', 'c'], dtype=dtype_target)
@@ -176,23 +177,23 @@ def test_str_cat_categorical(self, box, dtype_caller, dtype_target):
  # Series/Index with unaligned Index
  with tm.assert_produces_warning(expected_warning=FutureWarning):
  # FutureWarning to switch to alignment by default
- result = s.str.cat(t)
+ result = s.str.cat(t, sep=sep)
  assert_series_or_index_equal(result, expected)
 
  # Series/Index with Series having matching Index
  t = Series(t, index=s)
- result = s.str.cat(t)
+ result = s.str.cat(t, sep=sep)
  assert_series_or_index_equal(result, expected)
 
  # Series/Index with Series.values
- result = s.str.cat(t.values)
+ result = s.str.cat(t.values, sep=sep)
  assert_series_or_index_equal(result, expected)
 
  # Series/Index with Series having different Index
  t = Series(t.values, index=t)
  with tm.assert_produces_warning(expected_warning=FutureWarning):
  # FutureWarning to switch to alignment by default
- result = s.str.cat(t)
+ result = s.str.cat(t, sep=sep)
  assert_series_or_index_equal(result, expected)
 
  @pytest.mark.parametrize('box', [Series, Index])