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:
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
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:
def test_function():
with Replacer() as replace:
y = replace('testfixtures.tests.sample1.X.y', Mock())
y.return_value = 'mock y'
aMethod = replace('testfixtures.tests.sample1.X.aMethod', Mock())
aMethod.return_value = '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', Mock())
@replace('testfixtures.tests.sample1.X.aMethod', Mock())
def test_function(aMethod, y):
print(aMethod, y)
aMethod().return_value = 'mock method'
y().return_value = 'mock y'
x = X()
print(aMethod, y)
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.
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.
For example, suppose you have a data structure like the following:
You can mock out the value associated with key
and the second
element in the complex_key
list as follows:
from pprint import pprint
from testfixtures import Replacer
from testfixtures.tests.sample1 import some_dict
def test_function():
with Replacer() as replace:
replace('testfixtures.tests.sample1.some_dict.key', 'foo')
replace('testfixtures.tests.sample1.some_dict.complex_key.1', 42)
pprint(some_dict)
While the replacement is in effect, the new items are in place:
>>> test_function()
{'complex_key': [1, 42, 3], 'key': 'foo'}
When it is no longer in effect, the originals are returned:
>>> pprint(some_dict)
{'complex_key': [1, 2, 3], 'key': 'value'}
If the dictionary is os.environ
, then see Replacing environment variables.
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:
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('testfixtures.tests.sample1.some_dict.key', 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('testfixtures.tests.sample1.some_dict', 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!