Support assert statements inside validators (#653)

* Support assert statements inside validators

* Add a validator example that uses assert

* Add warning about consequences of using -O optimization flag

* Fix a typo

* Fix incomplete validator

* Extend exception name generation

* Improve tests

* Clarify pytest behaviour

* handle assertion error name, fix build

* Address feedback

* docs cleanup

* Incorporate feedback

* fix quotes

docs/examples/validators_simple.py

@@ -3,6 +3,7 @@ from pydantic import BaseModel, ValidationError, validator
 
 class UserModel(BaseModel):
     name: str
+    username: str
     password1: str
     password2: str
 
@@ -18,6 +19,11 @@ class UserModel(BaseModel):
             raise ValueError('passwords do not match')
         return v
 
+    @validator('username')
+    def username_alphanumeric(cls, v):
+        assert v.isalpha(), 'must be alphanumeric'
+        return v
+
 
 print(UserModel(name='samuel colvin', password1='zxcvbn', password2='zxcvbn'))
 # > UserModel name='Samuel Colvin' password1='zxcvbn' password2='zxcvbn'