Improve error messages

Author: topper-123

pandas/core/dtypes/dtypes.py

@@ -9,6 +9,7 @@
 from pandas._libs.tslibs import NaT, Period, Timestamp, timezones
 
 from pandas.core.dtypes.generic import ABCCategoricalIndex, ABCIndexClass
+from .inference import is_list_like
 
 from pandas import compat
 
@@ -403,7 +404,10 @@ def validate_categories(categories, fastpath=False):
         """
         from pandas import Index
 
-        if not isinstance(categories, ABCIndexClass):
+        if not fastpath and not is_list_like(categories, allow_sets=True):
+            msg = "Parameter 'categories' must be list-like, was {!r}"
+            raise TypeError(msg.format(categories))
+        elif not isinstance(categories, ABCIndexClass):
             categories = Index(categories, tupleize_cols=False)
 
         if not fastpath:

pandas/tests/dtypes/test_dtypes.py

@@ -94,7 +94,7 @@ def test_construction_from_string(self):
             TypeError, lambda: CategoricalDtype.construct_from_string('foo'))
 
     def test_constructor_invalid(self):
-        msg = "CategoricalIndex.* must be called"
+        msg = "categories must be list-like"
         with pytest.raises(TypeError, match=msg):
             CategoricalDtype("category")
 
@@ -710,7 +710,7 @@ def test_invalid_raises(self):
         with pytest.raises(TypeError, match='ordered'):
             CategoricalDtype(['a', 'b'], ordered='foo')
 
-        with pytest.raises(TypeError, match='collection'):
+        with pytest.raises(TypeError, match='categories must be list-like'):
             CategoricalDtype('category')
 
     def test_mixed(self):