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# Mocking out objects and methods

Mocking is the process of replacing objects used in your code with ones that make testing easier, but only while the tests are running.

This may mean replacing resources or dependencies, such as database connections or file paths, with ones that are isolated for testing. It may also mean replacing chunks of complex functionality that aren’t the subject of the test with mock objects that allow you to check that the mocked out functionality is being used as expected.

## What to mock with

Python has a standard mock implementation in the form of `unittest.mock` which is also available as a rolling backport so that the latest features and bugfixes can be used in any version of Python.

For convenience, testfixtures provides a facade over both of these in the form of `testfixtures.mock`. The contents are identical and preference is given to the rolling backport if it is present. The facade also contains any bugfixes that are critical to the operation of functionality provided by testfixtures.

Testfixtures also provides specialised mocks for dealing with dates and times and subprocesses.

## How to mock

Testfixtures provides `Replace`, `Replacer` and the `replace()` decorator to mock out objects. These work in a similar way to `unittest.mock.patch()`, and have been around longer. They still provide a little more flexibility than `patch()`, so use whichever feels best in your codebase.

## Methods of replacement

When using the tools provided by Testfixtures, there are three different methods of mocking out functionality that can be used to replace functions, classes or even individual methods on a class. Consider the following module:

testfixtures.tests.sample1

    class X:

        def y(self):
            return "original y"

        @classmethod
        def aMethod(cls):
            return cls

        @staticmethod
        def bMethod():
            return 2

We want to mock out the `y` method of the `X` class, with, for example, the following function:

    def mock_y(self):
         return 'mock y'

### The context managers

For replacement of a single thing, it’s easiest to use the `Replace` context manager:

    from testfixtures import Replace

    def test_function():
        with Replace('testfixtures.tests.sample1.X.y', mock_y):
            print(X().y())

For the duration of the `with` block, the replacement is used:

    >>> test_function()
    mock y

For multiple replacements, the `Replacer` context manager can be used instead:

    from testfixtures.mock import Mock
    from testfixtures import Replacer

    def test_function():
        with Replacer() as replace:
            mock_y = replace('testfixtures.tests.sample1.X.y', Mock())
            mock_y.return_value = 'mock y'
            print(X().y())

For the duration of the `with` block, the replacement is used:

    >>> test_function()
    mock y

You can also use explict relative traversal from an object, which is more friendly to static analysis tools such as IDEs:

    from testfixtures import Replace
    from testfixtures.tests.sample1 import X

    def test_function():
        with Replace(container=X, target='.y', replacement=mock_y):
            print(X().y())

For the duration of the `with` block, the replacement is used:

    >>> test_function()
    mock y

For replacements that are friendly to static analysis tools such as IDEs, three convenience context managers are provided:

- To replace or remove environment variables, use `replace_in_environ()`:

      import os
      from testfixtures import replace_in_environ

      def test_function():
          with replace_in_environ('SOME_ENV_VAR', 1234):
              print(repr(os.environ['SOME_ENV_VAR']))

  For the duration of the `with` block, the replacement is used:

      >>> test_function()
      '1234'

  For more details, see Replacing environment variables.

- To replace methods on classes, including normal methods, class methods and static methods, use `replace_on_class()`:

      from testfixtures import replace_on_class

      class MyClass:

          def the_method(self, value: str) -> str:
              return 'original' + value

      instance = MyClass()

      def test_function():
          with replace_on_class(
              MyClass.the_method,
              lambda self, value: type(self).__name__+value
          ):
              print(instance.the_method(':it'))

  For the duration of the `with` block, the replacement is used:

      >>> test_function()
      MyClass:it

  For more details, see Replacing methods on classes.

- To replace functions in modules use `replace_in_module()`:

      from testfixtures import replace_in_module
      from testfixtures.tests.sample1 import z as original_z

      def test_function():
          with replace_in_module(original_z, lambda: 'replacement z'):
              from testfixtures.tests.sample1 import z
              print(z())

  For the duration of the `with` block, the replacement is used:

      >>> test_function()
      replacement z

  For more details, see Replacing items in modules.

### The decorator

If you want to replace different things in different test functions, you may find the decorator suits your needs better:

    from testfixtures import replace

    @replace('testfixtures.tests.sample1.X.y', mock_y)
    def test_function():
        print(X().y())

When using the decorator, the replacement is used for the duration of the decorated callable’s execution:

    >>> test_function()
    mock y

If you need to manipulate or inspect the object that’s used as a replacement, you can add an extra parameter to your function. The decorator will see this and pass the replacement in it’s place:

    from testfixtures.mock import Mock, call
    from testfixtures import compare, replace

    @replace('testfixtures.tests.sample1.X.y', Mock())
    def test_function(mocked_y):
        mocked_y.return_value = 'mock y'
        print(X().y())
        compare(mocked_y.mock_calls, expected=[call()])

The above still results in the same output:

    >>> test_function()
    mock y

Note

This method is not compatible with pytest’s fixture discovery stuff. Instead, put a fixture such as the following in your `conftest.py`:

    from testfixtures import Replace
    import pytest

    @pytest.fixture()
    def mocked_y():
        m = Mock()
        with Replace('testfixtures.tests.sample1.X.y', m):
            yield m

### Manual usage

If you want to replace something for the duration of a doctest or you want to replace something for every test in a `TestCase`, then you can use the `Replacer` manually.

The instantiation and replacement are done in the set-up step of the `TestCase` or equivalent:

    >>> from testfixtures import Replacer
    >>> replacer = Replacer()
    >>> replacer.replace('testfixtures.tests.sample1.X.y', mock_y)

The replacement then stays in place until removed:

    >>> X().y()
    'mock y'

Then, in the tear-down step of the `TestCase` or equivalent, the replacement is removed:

    >>> replacer.restore()
    >>> X().y()
    'original y'

The `restore()` method can also be added as an `addCleanup()` if that is easier or more compact in your test suite.

## Replacing more than one thing

Both the `Replacer` and the `replace()` decorator can be used to replace more than one thing at a time. For the former, this is fairly obvious:

    from testfixtures.tests.sample1 import X

    def test_function():
        with Replacer() as replace:
            replace.on_class(X.y, lambda self: 'mock y')
            replace.on_class(X.aMethod, lambda cls: 'mock method')
            x = X()
            print(x.y(), x.aMethod())

For the decorator, it’s less obvious but still pretty easy:

    from testfixtures import replace

    @replace('testfixtures.tests.sample1.X.y', lambda self: 'mock y')
    @replace('testfixtures.tests.sample1.X.aMethod', lambda cls: 'mock method')
    def test_function(aMethod, y):
        print(aMethod, y)
        x = X()
        print(x.y(), x.aMethod())

You’ll notice that you can still get access to the replacements, even though there are several of them.

## Replacing things that may not be there

The following code shows a situation where `hpy` may or may not be present depending on whether the `guppy` package is installed or not.

testfixtures.tests.sample2

    try:
        from guppy import hpy
        guppy = True
    except ImportError:
        guppy = False

    def dump(path):
        if guppy:
            hpy().heap().stat.dump(path)

To test the behaviour of the code that uses `hpy` in both of these cases, regardless of whether or not the `guppy` package is actually installed, we need to be able to mock out both `hpy` and the `guppy` global. This is done by doing non-strict replacement, as shown in the following `TestCase`:

    from testfixtures.tests.sample2 import dump
    from testfixtures import replace
    from testfixtures.mock import Mock, call

    class Tests(unittest.TestCase):

        @replace('testfixtures.tests.sample2.guppy', True)
        @replace('testfixtures.tests.sample2.hpy', Mock(), strict=False)
        def test_method(self, hpy):

            dump('somepath')

            compare([
                     call(),
                     call().heap(),
                     call().heap().stat.dump('somepath')
                   ], hpy.mock_calls)

        @replace('testfixtures.tests.sample2.guppy', False)
        @replace('testfixtures.tests.sample2.hpy', Mock(), strict=False)
        def test_method_no_heapy(self,hpy):

            dump('somepath')

            compare(hpy.mock_calls,[])

Non-strict replacement using the `strict` keyword parameter is supported both when calling a `Replacer` or using the `replace()` method.

## Replacing items in dictionaries and lists

`Replace`, `Replacer` and the `replace()` decorator can be used to replace items in dictionaries and lists.

If the dictionary is `os.environ`, then see Replacing environment variables.

For a lists such as this:

    >>> sample_list = [1, 2, 3]

An element can be placed as follows:

    >>> with Replace(container=sample_list, target='.1', replacement=42):
    ...     print(sample_list)
    [1, 42, 3]

For dictionaries such as this:

    >>> sample_dict = {1: 'a', 'z': 'b'}

String keys can be replaced as follows:

    >>> with Replace(container=sample_dict, target='.z', replacement='c'):
    ...     print(sample_dict)
    {1: 'a', 'z': 'c'}

For non-string keys, it takes a bit more work:

    >>> from operator import getitem
    >>> with Replace(
    ...     container=sample_dict, accessor=getitem, name=1, target='a', replacement='c'
    ... ):
    ...     print(sample_dict)
    {1: 'c', 'z': 'b'}

For nested data structures such as this:

    >>> nested = {'key': [1, 2, 3]}

Nested traversal can be used:

    >>> with Replace(container=nested, target='.key.2', replacement=42):
    ...     print(nested)
    {'key': [1, 2, 42]}

If your dictionary or other item-based traversal key contains periods:

    >>> sample_dict = {'.foo': 'bar'}

You can use a different separator:

    >>> with Replace(container=sample_dict, target=':.foo', sep=':', replacement='baz'):
    ...     print(sample_dict)
    {'.foo': 'baz'}

## Removing attributes and dictionary items

`Replace`, `Replacer` and the `replace()` decorator can be used to remove attributes from objects and remove items from dictionaries.

For example, suppose you have a data structure like the following:

testfixtures.tests.sample1

    some_dict = dict(
        key='value',
        complex_key=[1, 2, 3],
    )

If you want to remove the `key` for the duration of a test, you can do so as follows:

    from testfixtures import Replace, not_there
    from testfixtures.tests.sample1 import some_dict

    def test_function():
        with Replace(container=some_dict, target='.key', replacement=not_there):
            pprint(some_dict)

While the replacement is in effect, `key` is gone:

    >>> test_function()
    {'complex_key': [1, 2, 3]}

When it is no longer in effect, `key` is returned:

    >>> pprint(some_dict)
    {'complex_key': [1, 2, 3], 'key': 'value'}

If you want the whole `some_dict` dictionary to be removed for the duration of a test, you would do so as follows:

    from testfixtures import Replace, not_there
    from testfixtures.tests import sample1

    def test_function():
        with Replace(container=sample1, target='.some_dict', replacement=not_there):
            print(hasattr(sample1, 'some_dict'))

While the replacement is in effect, `key` is gone:

    >>> test_function()
    False

When it is no longer in effect, `key` is returned:

    >>> pprint(sample1.some_dict)
    {'complex_key': [1, 2, 3], 'key': 'value'}

## Replacing environment variables

To ensure an environment variable is present and set to a particular value, use `in_environ()`:

    >>> import os
    >>> replace = Replacer()
    >>> replace.in_environ('SOME_ENV_VAR', 1234)
    >>> print(repr(os.environ['SOME_ENV_VAR']))
    '1234'

If you want to make sure an environment variable is unset and not present, use `not_there`:

    >>> replace.in_environ('SOME_ENV_VAR', not_there)
    >>> 'SOME_ENV_VAR' in os.environ
    False

## Replacing methods on classes

To replace methods on classes, including normal methods, class methods and static methods, in a way that is friendly to static analysis, use `on_class()`:

    class MyClass:

        def normal_method(self, value: str) -> str:
            return 'original' + value

        @classmethod
        def class_method(cls, value: str) -> str:
            return 'original' + value

        @staticmethod
        def static_method(value: str) -> str:
            return 'original' + value

For normal methods, the replacement will be called with the correct `self`:

    >>> instance = MyClass()
    >>> replace = Replacer()
    >>> replace.on_class(MyClass.normal_method, lambda self, value: type(self).__name__+value)
    >>> print(instance.normal_method(':it'))
    MyClass:it

For class methods, the replacement you provide will be wrapped in a `classmethod` if you have not already done so:

    >>> replace.on_class(MyClass.class_method, lambda cls, value: cls.__name__+value)
    >>> print(instance.class_method(':it'))
    MyClass:it

Likewise, for static methods, the replacement you provide will be wrapped in a `staticmethod` if you have not already done so:

    >>> replace.on_class(MyClass.static_method, lambda value: 'mocked'+value)
    >>> print(instance.static_method(':it'))
    mocked:it

If you need to replace a class attribute such as `FOO` in this example:

    class MyClass:
        FOO = 1

It can be done like this:

    >>> instance = MyClass()
    >>> replace = Replacer()
    >>> replace(MyClass.FOO, 42, container=MyClass, name='FOO')
    42
    >>> print(instance.FOO)
    42

If you encounter methods that have an incorrect `__name__`, such as those returned by poorly implemented decorators:

    def bad(f):
        def inner(self, x):
            return f(self, x)
        return inner

    class SampleClass:

        @bad
        def method(self, x):
            return x*2

They can be replaced by specifying the correct name:

    >>> instance = SampleClass()
    >>> replace = Replacer()
    >>> replace.on_class(SampleClass.method, lambda self, value: value*3, name='method')
    >>> print(instance.method(2))
    6

## Replacing items in modules

To replace functions in modules use `in_module()`:

    >>> from testfixtures.tests.sample1 import z as original_z
    >>> replace = Replacer()
    >>> replace.in_module(original_z, lambda: 'replacement z')
    >>> from testfixtures.tests.sample1 import z
    >>> z()
    'replacement z'

If you need to replace usage in a module other than the one where the function is defined, it can be done as follows

    >>> from testfixtures.tests.sample1 import z
    >>> from testfixtures.tests import sample3
    >>> replace = Replacer()
    >>> replace.in_module(z, lambda: 'replacement z', module=sample3)
    >>> sample3.z()
    'replacement z'

If you need to replace a module global, then you can use `Replace` as follows:

    >>> from testfixtures.tests import sample3
    >>> replacer = Replacer()
    >>> replacer.replace(sample3.SOME_CONSTANT, 43,
    ...                  container=sample3, name='SOME_CONSTANT')
    >>> from testfixtures.tests.sample3 import SOME_CONSTANT
    >>> SOME_CONSTANT
    43

## Gotchas

- Make sure you replace the object where it’s used and not where it’s defined. For example, with the following code from the `testfixtures.tests.sample1` package:

      from time import time

      def str_time():
          return str(time())

  You might be tempted to mock things as follows:

      >>> replace = Replacer()
      >>> replace('time.time', Mock())
      <...>

  But this won’t work:

      >>> from testfixtures.tests.sample1 import str_time
      >>> type(float(str_time()))
      <... 'float'>

  You need to replace `time()` where it’s used, not where it’s defined:

      >>> replace('testfixtures.tests.sample1.time', Mock())
      <...>
      >>> str_time()
      "<...Mock...>"

  A corollary of this is that you need to replace all occurrences of an original to safely be able to test. This can be tricky when an original is imported into many modules that may be used by a particular test.

- You can’t replace whole top level modules, and nor should you want to! The reason being that everything up to the last dot in the replacement target specifies where the replacement will take place, and the part after the last dot is used as the name of the thing to be replaced:

      >>> Replacer().replace('sys', Mock())
      Traceback (most recent call last):
       ...
      ValueError: target must contain at least one dot!