append dtype that's illegal (or has raised) in error message

Author: h-vetinari

pandas/core/strings.py

@@ -2291,9 +2291,11 @@ def _legal_dtype(series):
             legal = dtype == 'O' or (dtype == 'float' and series.isna().all())
             return legal
         err_wrong_dtype = ('Can only concatenate list-likes containing only '
-                           'strings (or missing values)')
+                           'strings (or missing values).')
         if any(not _legal_dtype(x) for x in others):
-            raise TypeError(err_wrong_dtype)
+            raise TypeError(err_wrong_dtype + ' Received list-like of dtype: '
+                            '{}'.format([x.dtype for x in others
+                                         if not _legal_dtype(x)][0]))
 
         if join is None and warn:
             warnings.warn("A future version of pandas will perform index "
@@ -2343,9 +2345,12 @@ def _legal_dtype(series):
                 # no NaNs - can just concatenate
                 result = cat_core(all_cols, sep)
         except TypeError as exc:
-            if re.match(r'can only concatenate str \(not \"\w+\"\) to str',
-                        str(exc)):
-                raise TypeError(err_wrong_dtype)
+            m = re.match(r'can only concatenate str \(not \"(\w+)\"\) to str',
+                         str(exc))
+            if m:
+                raise TypeError(err_wrong_dtype
+                                + ' Received list-like containing element of'
+                                + ' type: {}'.format(m.group(1)))
             raise exc
 
         if isinstance(self._orig, Index):