# Internals

Behind the scenes: steps performed when parsing a factory declaration, and when calling it.

This section will be based on the following factory declaration:

    class UserFactory(factory.Factory):
        class Meta:
            model = User

        class Params:
            # Allow us to quickly enable staff/superuser flags
            superuser = factory.Trait(
                is_superuser=True,
                is_staff=True,
            )
            # Meta parameter handling all 'enabled'-related fields
            enabled = True

        # Classic fields
        username = factory.Faker('user_name')
        full_name = factory.Faker('name')
        creation_date = factory.fuzzy.FuzzyDateTime(
            datetime.datetime(2000, 1, 1, tzinfo=datetime.timezone.utc),
            datetime.datetime(2015, 12, 31, 20, tzinfo=datetime.timezone.utc)
        )

        # Conditional flags
        is_active = factory.SelfAttribute('enabled')
        deactivation_date = factory.Maybe(
            'enabled',
            None,
            factory.fuzzy.FuzzyDateTime(
                datetime.datetime.now().replace(tzinfo=datetime.timezone.utc) - datetime.timedelta(days=10),
                datetime.datetime.now().replace(tzinfo=datetime.timezone.utc) - datetime.timedelta(days=1),
            ),
        )

        # Related logs
        creation_log = factory.RelatedFactory(
            UserLogFactory,
            factory_related_name='user',
            action='create',
            timestamp=factory.SelfAttribute('user.creation_date'),
        )

## Parsing, Step 1: Metaclass and type declaration

1. Python parses the declaration and calls (thanks to the metaclass declaration):

       factory.base.BaseFactory.__new__(
           'UserFactory',
           (factory.Factory,),
           attributes,
       )

2. That metaclass removes `Meta` and `Params` from the class attributes, then generate the actual factory class (according to standard Python rules)

3. It initializes a `FactoryOptions` object, and links it to the class

## Parsing, Step 2: adapting the class definition

1. The `FactoryOptions` reads the options from the `class Meta` declaration

2. It finds a few specific pointer (loading the model class, finding the reference factory for the sequence counter, etc.)

3. It copies declarations and parameters from parent classes

4. It scans current class attributes (from `vars()`) to detect pre/post declarations

5. Declarations are split among pre-declarations and post-declarations (a raw value shadowing a post-declaration is seen as a post-declaration)

Note

A declaration for `foo__bar` will be converted into parameter `bar` for declaration `foo`.

## Instantiating, Step 1: Converging entry points

First, decide the strategy:

- If the entry point is specific to a strategy (`build()`, `create_batch()`, …), use it

- If it is generic (`generate()`, `Factory.__call__()`), use the strategy defined at the `class Meta` level

Then, we’ll pass the strategy and passed-in overrides to the `Factory._generate` method.

Note

According to the project road map, a future version will use a `Factory._generate_batch` at its core instead.

A factory’s `Factory._generate` function actually delegates to a `StepBuilder()` object. This object will carry the overall “build an object” context (strategy, depth, and possibly other).

## Instantiating, Step 2: Preparing values

1. The `StepBuilder` merges overrides with the class-level declarations

2. The sequence counter for this instance is initialized

3. A `Resolver` is set up with all those declarations, and parses them in order; it will call each value’s `evaluate()` method, including extra parameters.

4. If needed, the `Resolver` might recurse (through the `StepBuilder`, e.g when encountering a `SubFactory`.

## Instantiating, Step 3: Building the object

1. The `StepBuilder` fetches the attributes computed by the `Resolver`.

2. It applies renaming/adjustment rules

3. It passes them to the `FactoryOptions.instantiate` method, which forwards to the proper methods.

4. Post-declaration are applied (in declaration order)

Note

This document discusses implementation details; there is no guarantee that the described methods names and signatures will be kept as is.