Small improvements to str_cat

pandas/core/strings.py

@@ -45,6 +45,15 @@ def str_cat(arr, others=None, sep=None, na_rep=None):
 
     Examples
     --------
+    When ``na_rep`` is `None` (default behavior), NaN value(s)
+    in the Series propagate and return value will be NaN.
+
+    >>> Series(['a','b',np.nan,'c']).str.cat(sep=' ')
+    nan
+
+    >>> Series(['a','b',np.nan,'c']).str.cat(sep=' ', na_rep='?')
+    'a b ? c'
+
     If ``others`` is specified, corresponding values are
     concatenated with the separator. Result will be a Series of strings.
 
@@ -110,11 +119,13 @@ def str_cat(arr, others=None, sep=None, na_rep=None):
 def _length_check(others):
     n = None
     for x in others:
-        if n is None:
-            n = len(x)
-        elif len(x) != n:
-            raise ValueError('All arrays must be same length')
-
+        try:
+            if n is None:
+                n = len(x)
+            elif len(x) != n:
+                raise ValueError('All arrays must be same length')
+        except TypeError:
+            raise ValueError("Did you mean to supply a `sep` keyword?")
     return n
 
 

pandas/tests/test_strings.py

@@ -126,6 +126,8 @@ def test_cat(self):
         exp = ['aa', NA, 'bb', 'bd', 'cfoo', NA]
         tm.assert_almost_equal(result, exp)
 
+
+
     def test_count(self):
         values = ['foo', 'foofoo', NA, 'foooofooofommmfoo']
 
@@ -2057,6 +2059,17 @@ def test_method_on_bytes(self):
                                        'S2').astype(object))
             tm.assert_series_equal(result, expected)
 
+    def test_str_cat_raises_intuitive_error(self):
+        s = Series(['a','b','c','d'])
+        message = "Did you mean to supply a `sep` keyword?"
+        with tm.assertRaisesRegexp(ValueError, message):
+            s.str.cat('|')
+        with tm.assertRaisesRegexp(ValueError, message):
+            s.str.cat('    ')
+
+
+
+
 
 if __name__ == '__main__':
     nose.runmodule(argv=[__file__, '-vvs', '-x', '--pdb', '--pdb-failure'],