better import errors, fix #309 (#336)

HISTORY.rst

@@ -14,6 +14,7 @@ v0.17.0 (unreleased)
 * support for passing Config class in dataclasses decorator, #276 by @jarekkar
   (**breaking change**: this supersedes the ``validate_assignment`` argument with ``config``)
 * support for nested dataclasses, #334 by @samuelcolvin
+* better errors when getting an ``ImportError`` with ``PyObject``, #309 by @samuelcolvin
 
 v0.16.1 (2018-12-10)
 ....................

pydantic/errors.py

@@ -109,7 +109,7 @@ class PathNotADirectoryError(_PathValueError):
 
 
 class PyObjectError(PydanticTypeError):
-    msg_template = 'ensure this value contains valid import path'
+    msg_template = 'ensure this value contains valid import path: {error_message}'
 
 
 class ListError(PydanticTypeError):

pydantic/types.py

@@ -6,7 +6,7 @@ from typing import Optional, Pattern, Set, Type, Union
 from uuid import UUID
 
 from . import errors
-from .utils import change_exception, import_string, make_dsn, url_regex_generator, validate_email
+from .utils import import_string, make_dsn, url_regex_generator, validate_email
 from .validators import (
     anystr_length_validator,
     anystr_strip_whitespace,
@@ -221,8 +221,10 @@ class PyObject:
     @classmethod
     def validate(cls, value):
         if value is not None:
-            with change_exception(errors.PyObjectError, ImportError):
+            try:
                 return import_string(value)
+            except ImportError as e:
+                raise errors.PyObjectError(error_message=str(e))
 
 
 class DSN(str):

tests/test_types.py

@@ -111,10 +111,27 @@ def test_module_import():
 
     m = PyObjectModel()
     assert m.module == os.path
+
     with pytest.raises(ValidationError) as exc_info:
         PyObjectModel(module='foobar')
     assert exc_info.value.errors() == [
-        {'loc': ('module',), 'msg': 'ensure this value contains valid import path', 'type': 'type_error.pyobject'}
+        {
+            'loc': ('module',),
+            'msg': 'ensure this value contains valid import path: ' '"foobar" doesn\'t look like a module path',
+            'type': 'type_error.pyobject',
+            'ctx': {'error_message': '"foobar" doesn\'t look like a module path'},
+        }
+    ]
+
+    with pytest.raises(ValidationError) as exc_info:
+        PyObjectModel(module='os.missing')
+    assert exc_info.value.errors() == [
+        {
+            'loc': ('module',),
+            'msg': 'ensure this value contains valid import path: ' 'Module "os" does not define a "missing" attribute',
+            'type': 'type_error.pyobject',
+            'ctx': {'error_message': 'Module "os" does not define a "missing" attribute'},
+        }
     ]