Add basic typing support

Only `Factory.build()` and `Factory.create()` are properly typed,
provided the class is declared as `class UserFactory(Factory[User]):`.

Relies on mypy for tests.

Reviewed-By: Raphaël Barrois <raphael.barrois@polytechnique.org>

Makefile

@@ -54,6 +54,7 @@ testall:
 # DOC: Run tests for the currently installed version
 # Remove cgi warning when dropping support for Django<=4.1.
 test:
+	mypy --ignore-missing-imports tests/test_typing.py
 	python \
 		-b \
 		-X dev \

factory/__init__.py

@@ -1,5 +1,7 @@
 # Copyright: See the LICENSE file.
 
+import sys
+
 from .base import (
     BaseDictFactory,
     BaseListFactory,
@@ -70,10 +72,10 @@
     pass
 
 __author__ = 'Raphaël Barrois <raphael.barrois+fboy@polytechnique.org>'
-try:
+if sys.version_info >= (3, 8):
     # Python 3.8+
     import importlib.metadata as importlib_metadata
-except ImportError:
+else:
     import importlib_metadata
 
 __version__ = importlib_metadata.version("factory_boy")

factory/base.py

@@ -4,11 +4,14 @@
 import collections
 import logging
 import warnings
+from typing import Generic, List, Type, TypeVar
 
 from . import builder, declarations, enums, errors, utils
 
 logger = logging.getLogger('factory.generate')
 
+T = TypeVar('T')
+
 # Factory metaclasses
 
 
@@ -405,7 +408,7 @@ def reset(self, next_value=0):
         self.seq = next_value
 
 
-class BaseFactory:
+class BaseFactory(Generic[T]):
     """Factory base support for sequences, attributes and stubs."""
 
     # Backwards compatibility
@@ -506,12 +509,12 @@ def _create(cls, model_class, *args, **kwargs):
         return model_class(*args, **kwargs)
 
     @classmethod
-    def build(cls, **kwargs):
+    def build(cls, **kwargs) -> T:
         """Build an instance of the associated class, with overridden attrs."""
         return cls._generate(enums.BUILD_STRATEGY, kwargs)
 
     @classmethod
-    def build_batch(cls, size, **kwargs):
+    def build_batch(cls, size: int, **kwargs) -> List[T]:
         """Build a batch of instances of the given class, with overridden attrs.
 
         Args:
@@ -523,12 +526,12 @@ def build_batch(cls, size, **kwargs):
         return [cls.build(**kwargs) for _ in range(size)]
 
     @classmethod
-    def create(cls, **kwargs):
+    def create(cls, **kwargs) -> T:
         """Create an instance of the associated class, with overridden attrs."""
         return cls._generate(enums.CREATE_STRATEGY, kwargs)
 
     @classmethod
-    def create_batch(cls, size, **kwargs):
+    def create_batch(cls, size: int, **kwargs) -> List[T]:
         """Create a batch of instances of the given class, with overridden attrs.
 
         Args:
@@ -627,18 +630,22 @@ def simple_generate_batch(cls, create, size, **kwargs):
         return cls.generate_batch(strategy, size, **kwargs)
 
 
-class Factory(BaseFactory, metaclass=FactoryMetaClass):
+class Factory(BaseFactory[T], metaclass=FactoryMetaClass):
     """Factory base with build and create support.
 
     This class has the ability to support multiple ORMs by using custom creation
     functions.
     """
 
+    # Backwards compatibility
+    AssociatedClassError: Type[Exception]
+
     class Meta(BaseMeta):
         pass
 
 
-# Backwards compatibility
+# Add the association after metaclass execution.
+# Otherwise, AssociatedClassError would be detected as a declaration.
 Factory.AssociatedClassError = errors.AssociatedClassError
 
 

factory/django.py

@@ -9,6 +9,7 @@
 import logging
 import os
 import warnings
+from typing import Dict, TypeVar
 
 from django.contrib.auth.hashers import make_password
 from django.core import files as django_files
@@ -20,9 +21,9 @@
 
 
 DEFAULT_DB_ALIAS = 'default'  # Same as django.db.DEFAULT_DB_ALIAS
+T = TypeVar("T")
 
-
-_LAZY_LOADS = {}
+_LAZY_LOADS: Dict[str, object] = {}
 
 
 def get_model(app, model):
@@ -72,7 +73,7 @@ def get_model_class(self):
         return self.model
 
 
-class DjangoModelFactory(base.Factory):
+class DjangoModelFactory(base.Factory[T]):
     """Factory for Django models.
 
     This makes sure that the 'sequence' field of created objects is a new id.

factory/faker.py

@@ -14,6 +14,7 @@ class Meta:
 
 
 import contextlib
+from typing import Dict
 
 import faker
 import faker.config
@@ -47,7 +48,7 @@ def evaluate(self, instance, step, extra):
         subfaker = self._get_faker(locale)
         return subfaker.format(self.provider, **extra)
 
-    _FAKER_REGISTRY = {}
+    _FAKER_REGISTRY: Dict[str, faker.Faker] = {}
     _DEFAULT_LOCALE = faker.config.DEFAULT_LOCALE
 
     @classmethod

setup.cfg

@@ -47,6 +47,7 @@ dev =
     Django
     flake8
     isort
+    mypy
     Pillow
     SQLAlchemy
     sqlalchemy_utils

tests/test_typing.py

@@ -0,0 +1,31 @@
+# Copyright: See the LICENSE file.
+
+import dataclasses
+import unittest
+
+import factory
+
+
+@dataclasses.dataclass
+class User:
+    name: str
+    email: str
+    id: int
+
+
+class TypingTests(unittest.TestCase):
+
+    def test_simple_factory(self) -> None:
+
+        class UserFactory(factory.Factory[User]):
+            name = "John Doe"
+            email = "john.doe@example.org"
+            id = 42
+
+            class Meta:
+                model = User
+
+        result: User
+        result = UserFactory.build()
+        result = UserFactory.create()
+        self.assertEqual(result.name, "John Doe")

tox.ini

@@ -35,6 +35,7 @@ passenv =
     POSTGRES_HOST
     POSTGRES_DATABASE
 deps =
+    mypy
     alchemy: SQLAlchemy
     alchemy: sqlalchemy_utils
     mongo: mongoengine