Add README.

Signed-off-by: Raphaël Barrois <raphael.barrois@polytechnique.org>

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+factory_boy
+===========
+
+factory_boy is a fixtures replacement based on thoughtbot's `factory_girl <http://github.com/thoughtbot/factory_girl>`_ . Like factory_girl it has a straightforward definition syntax, support for multiple build strategies (saved instances, unsaved instances, attribute dicts, and stubbed objects), and support for multiple factories for the same class, including factory inheritance. Django support is included, and support for other ORMs can be easily added.
+
+The official repository is at http://github.com/rbarrois/factory_boy.
+
+Credits
+-------
+
+This README parallels the factory_girl README as much as possible; text and examples are reproduced for comparison purposes. Ruby users of factory_girl should feel right at home with factory_boy in Python.
+
+factory_boy was originally written by Mark Sandstrom, and improved by Raphaël Barrois.
+
+Thank you Joe Ferris and thoughtbot for creating factory_girl.
+
+Download
+--------
+
+Github: http://github.com/rbarrois/factory_boy/
+
+PyPI::
+
+    pip install factory_boy
+
+Source::
+
+    # Download the source and run
+    python setup.py install
+
+
+Defining factories
+------------------
+
+Factories declare a set of attributes used to instantiate an object. The class of the object must be defined in the FACTORY_FOR attribute::
+
+    import factory
+    from models import User
+
+    class UserFactory(factory.Factory):
+        FACTORY_FOR = User
+
+        first_name = 'John'
+        last_name = 'Doe'
+        admin = False
+
+    # Another, different, factory for the same object
+    class AdminFactory(factory.Factory):
+        FACTORY_FOR = User
+
+        first_name = 'Admin'
+        last_name = 'User'
+        admin = True
+
+Using factories
+---------------
+
+factory_boy supports several different build strategies: build, create, attributes and stub::
+
+    # Returns a User instance that's not saved
+    user = UserFactory.build()
+
+    # Returns a saved User instance
+    user = UserFactory.create()
+
+    # Returns a dict of attributes that can be used to build a User instance
+    attributes = UserFactory.attributes()
+
+    # Returns an object with all defined attributes stubbed out:
+    stub = UserFactory.stub()
+
+You can use the Factory class as a shortcut for the default build strategy::
+
+    # Same as UserFactory.create()
+    user = UserFactory()
+
+The default strategy can be overridden::
+
+    UserFactory.default_strategy = factory.BUILD_STRATEGY
+    user = UserFactory()
+
+The default strategy can also be overridden for all factories::
+
+    # This will set the default strategy for all factories that don't define a default build strategy
+    factory.Factory.default_strategy = factory.BUILD_STRATEGY
+
+No matter which strategy is used, it's possible to override the defined attributes by passing keyword arguments::
+
+    # Build a User instance and override first_name
+    user = UserFactory.build(first_name='Joe')
+    user.first_name
+    # => 'Joe'
+
+Lazy Attributes
+---------------
+
+Most factory attributes can be added using static values that are evaluated when the factory is defined, but some attributes (such as associations and other attributes that must be dynamically generated) will need values assigned each time an instance is generated. These "lazy" attributes can be added as follows::
+
+    class UserFactory(factory.Factory):
+        first_name = 'Joe'
+        last_name = 'Blow'
+        email = factory.LazyAttribute(lambda a: '{0}.{1}@example.com'.format(a.first_name, a.last_name).lower())
+
+    UserFactory().email
+    # => 'joe.blow@example.com'
+
+The function passed to ``LazyAttribute`` is given the attributes defined for the factory up to the point of the LazyAttribute declaration. If a lambda won't cut it, the ``lazy_attribute`` decorator can be used to wrap a function::
+
+    # Stub factories don't have an associated class.
+    class SumFactory(factory.StubFactory):
+        lhs = 1
+        rhs = 1
+
+        @lazy_attribute
+        def sum(a):
+            result = a.lhs + a.rhs  # Or some other fancy calculation
+            return result
+
+Associations
+------------
+
+Associated instances can also be generated using ``LazyAttribute``::
+
+    from models import Post
+
+    class PostFactory(factory.Factory):
+        author = factory.LazyAttribute(lambda a: UserFactory())
+
+The associated object's default strategy is always used::
+
+    # Builds and saves a User and a Post
+    post = PostFactory()
+    post.id == None           # => False
+    post.author.id == None    # => False
+
+    # Builds and saves a User, and then builds but does not save a Post
+    post = PostFactory.build()
+    post.id == None           # => True
+    post.author.id == None    # => False
+
+Inheritance
+-----------
+
+You can easily create multiple factories for the same class without repeating common attributes by using inheritance::
+
+    class PostFactory(factory.Factory):
+        title = 'A title'
+
+    class ApprovedPost(PostFactory):
+        approved = True
+        approver = factory.LazyAttribute(lambda a: UserFactory())
+
+Sequences
+---------
+
+Unique values in a specific format (for example, e-mail addresses) can be generated using sequences. Sequences are defined by using ``Sequence`` or the decorator ``sequence``::
+
+    class UserFactory(factory.Factory):
+        email = factory.Sequence(lambda n: 'person{0}@example.com'.format(n))
+
+    UserFactory().email  # => 'person0@example.com'
+    UserFactory().email  # => 'person1@example.com'
+
+Sequences can be combined with lazy attributes::
+
+    class UserFactory(factory.Factory):
+        name = 'Mark'
+        email = factory.LazyAttributeSequence(lambda a, n: '{0}+{1}@example.com'.format(a.name, n).lower())
+
+    UserFactory().email  # => mark+0@example.com
+
+If you wish to use a custom method to set the initial ID for a sequence, you can override the ``_setup_next_sequence`` class method::
+
+    class MyFactory(factory.Factory):
+
+        @classmethod
+        def _setup_next_sequence(cls):
+            return cls._associated_class.objects.values_list('id').order_by('-id')[0] + 1
+
+Customizing creation
+--------------------
+
+Sometimes, the default build/create by keyword arguments doesn't allow for enough
+customization of the generated objects. In such cases, you should override the
+Factory._prepare method::
+
+    class UserFactory(factory.Factory):
+        @classmethod
+        def _prepare(cls, create, **kwargs):
+            password = kwargs.pop('password', None)
+            user = super(UserFactory, cls)._prepare(create, **kwargs)
+            if password:
+                user.set_password(password)
+                if create:
+                    user.save()
+            return user
+
+Subfactories
+------------
+
+If one of your factories has a field which is another factory, you can declare it as a ``SubFactory``. This allows to define attributes of that field when calling
+the global factory, using a simple syntax : ``field__attr=42`` will set the attribute ``attr`` of the ``SubFactory`` defined in ``field`` to 42::
+
+    class InnerFactory(factory.Factory):
+        foo = 'foo'
+        bar = factory.LazyAttribute(lambda o: foo * 2)
+
+    class ExternalFactory(factory.Factory):
+        inner = factory.SubFactory(InnerFactory, foo='bar')
+
+    >>> e = ExternalFactory()
+    >>> e.foo
+    'bar'
+    >>> e.bar
+    'barbar'
+
+    >>> e2 : ExternalFactory(inner__bar='baz')
+    >>> e2.foo
+    'bar'
+    >>> e2.bar
+    'baz'
+
+