Print exceptions raised from from_param methods

This makes debugging the problems easier.

Change-Id: I285d8176b8cfb0cdf1e9cecd8ae5a0a7f0f109e7

tests/pecantest/test/tests/test_ws.py

@@ -91,7 +91,8 @@ class TestWS(FunctionalTest):
         self.assertEqual(
             a['faultstring'],
             "Invalid input for field/attribute author_id. "
-            "Value: 'foobar'. unable to convert to int")
+            "Value: 'foobar'. unable to convert to int. Error: invalid "
+            "literal for int() with base 10: 'foobar'")
 
     def test_clientsideerror(self):
         expected_status_code = 400

wsme/rest/args.py

@@ -178,7 +178,7 @@ def args_from_args(funcdef, args, kwargs):
     for argdef, arg in zip(funcdef.arguments[:len(args)], args):
         try:
             newargs.append(from_param(argdef.datatype, arg))
-        except Exception:
+        except Exception as e:
             if isinstance(argdef.datatype, UserType):
                 datatype_name = argdef.datatype.name
             elif isinstance(argdef.datatype, type):
@@ -188,7 +188,8 @@ def args_from_args(funcdef, args, kwargs):
             raise InvalidInput(
                 argdef.name,
                 arg,
-                "unable to convert to %s" % datatype_name)
+                "unable to convert to %(datatype)s. Error: %(error)s" % {
+                    'datatype': datatype_name, 'error': e})
     newkwargs = {}
     for argname, value in kwargs.items():
         newkwargs[argname] = from_param(

wsme/tests/test_protocols_commons.py

@@ -93,6 +93,31 @@ class TestProtocolsCommons(unittest.TestCase):
         else:
             self.fail('Should have thrown an InvalidInput')
 
+    def test_args_from_args_custom_exc(self):
+
+        class FakeType(UserType):
+            name = 'fake-type'
+            basetype = int
+
+            def validate(self, value):
+                if value < 10:
+                    raise ValueError('should be greater than 10')
+
+            def frombasetype(self, value):
+                self.validate(value)
+
+        fake_type = FakeType()
+        fd = FunctionDefinition(FunctionDefinition)
+        fd.arguments.append(FunctionArgument('fake-arg', fake_type, True, 0))
+
+        try:
+            args_from_args(fd, [9], {})
+        except InvalidInput as e:
+            assert fake_type.name in str(e)
+            assert 'Error: should be greater than 10' in str(e)
+        else:
+            self.fail('Should have thrown an InvalidInput')
+
     def test_args_from_args_array_type(self):
         fake_type = ArrayType(MyBaseType)
         fd = FunctionDefinition(FunctionDefinition)