Improve error message for unknown generics.

CHANGELOG.rst

@@ -1,3 +1,12 @@
+2019.12.18
+--------------
+
+Changes
+^^^^^^^
+
+- Improve error message for unknown generics.
+  `#10 <https://github.com/python-desert/desert/pull/10>`_
+
 2019.12.10
 --------------
 

src/desert/_make.py

@@ -102,6 +102,12 @@ def class_schema(clazz: type, meta: Dict[str, Any] = {}) -> Type[marshmallow.Sch
     """
 
     fields: Union[Tuple[dataclasses.Field], Tuple[attr.Attribute]]
+
+    if not isinstance(clazz, type):
+        raise desert.exceptions.UnknownType(
+            f"Desert failed to infer the field type for {clazz}.\n"
+            + "Explicitly pass a Marshmallow field type."
+        )
     if dataclasses.is_dataclass(clazz):
         fields = dataclasses.fields(clazz)
     elif attr.has(clazz):

tests/test_make.py

@@ -1,6 +1,7 @@
 import dataclasses
 import datetime
 import enum
+import sys
 import types
 import typing as t
 
@@ -355,3 +356,17 @@ class A:
 
     with pytest.raises(desert.exceptions.UnknownType):
         desert.schema_class(A)
+
+
+@pytest.mark.skipif(
+    sys.version_info[:2] <= (3, 6), reason="3.6 has isinstance(t.Sequence[int], type)."
+)
+def test_raise_unknown_generic(module):
+    """Raise UnknownType for unknown generics."""
+
+    @module.dataclass
+    class A:
+        x: t.Sequence[int]
+
+    with pytest.raises(desert.exceptions.UnknownType):
+        desert.schema_class(A)

tox.ini

@@ -85,7 +85,7 @@ skip_install = true
 commands =
     coverage report
     coverage html
-    cuv graph
+
 
 [testenv:clean]
 commands = coverage erase