==================
 Patch Decorators
==================


.. currentmodule:: mock


The patch decorators are used for patching objects only within the scope of
the function they decorate. They automatically handle the unpatching for you,
even if exceptions are raise. All of these functions can also be used in with
statements.


patch
=====

.. function:: patch(target, new=None, spec=None, create=False)

``patch`` decorates a function. Inside the body of the function, the
``target`` (specified in the form 'PackageName.ModuleName.ClassName') is
patched with a ``new`` object. When the function exits the patch is undone.

The target is imported and the specified attribute patched with the new object
- so it must be importable from the environment you are calling the decorator
from.

If ``new`` is omitted, then a new ``Mock`` is created and passed in as an
extra argument to the decorated function::

   @patch('Package.ModuleName.ClassName')
   def test_something(self, MockClass):
       "test something"

The ``spec`` keyword argument is passed to the ``Mock`` if patch is creating
one for you.

In addition you can pass ``spec=True``, which causes patch to pass in the
object being mocked as the spec object. If you are using patch to mock out a
class, then the object you are interested in will probably be the return value
of the Mock (the instance it returns when called). Because of this, if you use
'spec=True' and are patching a class (and having patch create a Mock for you)
then the object being patched will be used as a spec for both the Mock *and*
its return value.

Using the class as a spec object for the created Mock (and return value) means
that the Mock will raise an exception if the code attempts to access any
attributes that don't exist. ::

    @patch('Package.ModuleName.ClassName', spec=True)
    def test_something(self, MockClass):
        instance = ClassName()
        self.assertRaises(AttributeError, lambda: instance.fake_attribute)

Patch can be used with the with statement - if this is available in your
version of Python. Here the patching applies to the indented block after the
with statement. Note that the patched object can always appear after the "as"
- even if an object to be patched was specified, though it can be omitted. ::

    with patch('Package.ModuleName.ClassName', spec=True) as MockClass:
        instance = ClassName()
        self.assertRaises(AttributeError, lambda: instance.fake_attribute)


By default ``patch`` will fail to replace attributes that don't exist. If you
pass in 'create=True' and the attribute doesn't exist, patch will create the
attribute for you when the patched function is called, and delete it again
afterwards. This is useful for writing tests against attributes that your
production code creates at runtime. It is off by by default because it can be
dangerous. With it switched on you can write passing tests against APIs that
don't actually exist!

Patch can be also used as a TestCase class decorator. It works by decorating each test method in the class. This reduces the boilerplate code when your test methods share a common patchings set. ::

  @patch('Patch.ModuleName.ClassName')
  class SomeTest(unittest.TestCase):
      def test_something(self, MockClass):
          "test something"

patch.object
============

.. function:: patch.object(target, attribute, new=None, spec=None, create=False)

    ``patch.object`` patches named members on objects - usually class or
    module objects.

You can either call it with three arguments or two arguments. The three
argument form takes the object to be patched, the attribute name and the
object to replace the attribute with.

When calling with the two argument form you omit the replacement object, and a
mock is created for you and passed in as an extra argument to the decorated
function::

    @patch.object(SomeClass, 'classmethod')
    def test_something(self, mockMethod):
        SomeClass.classmethod(3)
        
        mockMethod.assert_called_with(3) 

``spec`` and ``create`` have the same meaning as for the patch decorator.

``patch.object`` is also a context manager and can be used with ``with``
statements in the same way as ``patch``. It can also be used as a class
decorator with same semantics as ``patch``.

patch_object
============

.. deprecated:: 0.7
   This is the same as ``patch.object``. Use the renamed version now.


Nesting Patch Decorators
========================

If you want to perform multiple patches then you can simply stack up the
decorators.

You can stack up multiple patch decorators using this pattern::

    @patch('module.ClassName1')
    @patch('module.ClassName2')
    def testMethod(self, MockClass2, MockClass1):
        ClassName1()
        ClassName2()
        self.assertEqual(MockClass1.called, "ClassName1 not patched")
        self.assertEqual(MockClass2.called, "ClassName2 not patched")


Like all context-managers patches can be nested using contextlib's nested
function - *every* patching will appear in the tuple after "as". ::

     from contextlib import nested
     with nested(patch('Package.ModuleName.ClassName'), 
                 patch('Package.ModuleName.ClassName2', TestUtils.MockClass2)) as (MockClass1, MockClass2):
          instance = ClassName(ClassName2())
          self.assertEqual(instance.f(), "expected")


Patching Descriptors
====================

Since version 0.6.0 both patch_ and patch.object_ have been able to correctly
patch and restore descriptors; class methods, static methods and properties.
You should patch these on the *class* rather than an instance::

    @patch('module.ClassName.static')
    def testMethod(self, mockStatic):
        ClassName.static('foo')
        mockStatic.assert_called_with('foo')
