known-side-effects

Known Side Effects

A test utility library to help write explict side effects for mocked objects.

Mocks side effects are manipulated by when and then functions.

mock.when(...).then(...)

To enable this extend the mock class in you tests.

import known_side_effects
from unittest.mock import Mock
...
known_side_effects.extend(Mock)

When

All parameters in the when function are used to define the expected parameters for the side effect.

mock.when('argument_one', arg='argument_two').then(...)

If the mock is called with parameters that don't match any of the specified parameter sets then an UnmatchedArguments exception is raised. The arguments have to match exactly.

E.g. Given mock.when('argument_one', arg='argument_two').then(...) when the mock is called with the parameters in the table below an UnmatchedArguments is either raised or not raised.

Parameters	Raises
mock('argument_one', arg='argument_two')	False
mock(arg='argument_two')	True
mock('argument_one')	True

Multiple sets of parameters to match can be specified.

mock.when('first_specified_argument').then(...)
mock.when('second_specified_argument').then(...)
mock.when('third_specified_argument').then(...)

Chaining

Calling the when function with exactly the same arguments will allow you to append to the responses.

mock.when('arg').then('response_one')
mock.when('arg').then('response_two')

is the same as

mock.when('arg').then('response_one').then('response_two')

Then

The then function specifies what the known side effect should do when parameters are matched. By default it will just return what has been passed into the then function.

from unittest.mock import Mock
...
response_one = Mock()
mock = Mock()

mock.when(...).then(response_one)

assert mock(...) == response_one

To raise an exception rather that return a value call then_raise rather than then. An exception will be raised instead of returned.

from unittest.mock import Mock
...
exception = Exception()
mock = Mock()

mock.when(...).then_raise(exception)

mock(...)  # Raises the exception

You can also chain the then functions to return multiple different reponses. Each response will be returned once until the last response. Once the last response is reached then that reponse will be the only thing returned.

from unittest.mock import Mock
...
exception = Exception()
response_one = Mock()
mock = Mock()

mock.when(...).then(response_one).then_raise(exception)

assert mock(...) == response_one
mock(...)  # Raises the exception
mock(...)  # Raises the exception

Otherwise

You can specify default return values on a mock by calling otherwise. If the mock is called without matching any arguments then the otherwise value will returned.

mock.when('arg').then(...).otherwise('otherwise')

assert mock('not arg') == 'otherwise'

You can also raise an exception by default

mock.when(...).then(...).otherwise_raise(Exception())

Always

You can specify the mock to always return the same response regardless of what arguments it is called with.

mock.when().always('response')

assert mock(...) == 'response'

You can also raise an exception

mock.when().always_raise(Exception())

Reset

You can reset the the known side effects on a mock by passing it the reset function.

from unittest.mock import Mock
from known_side_effects import reset
...

mock = Mock()

mock.when(...).then(...)

reset(mock)

mock(...)  # raises an UnmatchedArguments exception

Gotcha

When calling the mock after specifying multiple known side effects, the first matched set of parameters will be executed. The order of matching is the order that the known side effects are defined in. If multiple arguments are specified where one matches a super set of the other (see Matchers) then the first matched will be executed. e.g.

from unittest.mock import Mock
from known_side_effects import AnyArg
...
response_one = Mock()
response_two = Mock()
argument_one = Mock()
argument_two = Mock()

mock.when(argument_one).then(response_one)
mock.when(AnyArg()).then(response_two)
...
assert mock(argument_one) == response_one
assert mock(argument_two) == response_two

If the order of the known side effects were reversed, the mock would only ever return response_two. This is due to the fact that the Any matched will match all parameters, therefore never attempting to match argument_one as it has already found a match. e.g.

from unittest.mock import Mock
from known_side_effects import AnyArg
...
response_one = Mock()
response_two = Mock()
argument_one = Mock()
argument_two = Mock()

mock.when(AnyArg()).then(response_two)        # These two lines have swapped
mock.when(argument_one).then(response_one) # These two lines have swapped

...
# This will raise an AssertionError as calling the mock with argument_one now
# returns response_two and not response_one
assert mock(argument_one) == response_one   
assert mock(argument_two) == response_two

Matchers

Matchers can be passed to known side effects as parameters. They are implementations of hamcrest matchers. Matchers will only match a single parameter.