python check if a method is called without mocking it away

Question

class A():
    def tmp(self):
        print("hi")

def b(a):
    a.tmp()

To check if tmp method is called in b, the recommended way is

a = A()
a.tmp = MagicMock()
b(a)
a.tmp.assert_called()

But tmp here is being mocked away and is not resulting in a "hi" getting printed.

I would want my unit test to check if method tmp is called without mocking it away.

Is this possible?

I know this is not a standard thing to expect when writing unitests. But my use case (which is bit tricky) requires this.

Patrick Haugh · Accepted Answer · 2018-06-21T14:01:37.277

41

You can set the Mock.side_effect to be the original method.

from unittest.mock import MagicMock

class A():
    def tmp(self):
        print("hi")

def b(a):
    a.tmp()

a = A()
a.tmp = MagicMock(side_effect=a.tmp)
b(a)
a.tmp.assert_called()

When side_effect is a function (or a bound method in this case, which is a kind of function), calling the Mock will also call the side_effect with the same arguments.

The Mock() call will return whatever the side_effect returns, unless it returns the unnittest.mock.DEFAULT singleton. Then it will return Mock.return_value instead.

edited Jun 21 '18 at 14:01

answered Jun 21 '18 at 13:55

Patrick Haugh

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1

def tmp(self): return "hi" – claudius Jun 21 '18 at 13:59
what if I care about return value as well? – claudius Jun 21 '18 at 13:59
1

@claudius Did you try it? Calling the `Mock` object should return whatever `side_effect` returns with those arguments. – Patrick Haugh Jun 21 '18 at 14:02
import should be from mock import MagicMock :) – Panduranga Rao Sadhu Jun 22 '20 at 13:06
What if you don't have access to the A object instance? – cowlinator Jan 24 '23 at 02:51
@cowlinator It depends on how everything is set up, you should probably ask a new question so you can include all the relevant details. – Patrick Haugh Jan 24 '23 at 05:17

score 2 · Answer 2 · answered Jun 21 '18 at 13:58

Or you can decorate the method to test:

def check_called(fun):
    def wrapper(self, *args, **kw):
        attrname = "_{}_called".format(fun.__name__)
        setattr(self, attrname, True)
        return fun(self, *args, **kw)
    return wrapper


a = A()
a.tmp = check_called(a.tmp)
b(a)
assert(getattr(a, "_tmp_called", False))

but MagicMock's side_effect is definitly a better solution if you're already using Mock ;)

score 0 · Answer 3 · answered Mar 22 '22 at 10:29

In addition to @Patrick Haugh's answer, you can also pass the function to the wraps argument (and it seems more semantically correct to me).

wraps: Item for the mock object to wrap. If wraps is not None then calling the Mock will pass the call through to the wrapped object (returning the real result). Attribute access on the mock will return a Mock object that wraps the corresponding attribute of the wrapped object (so attempting to access an attribute that doesn’t exist will raise an AttributeError).

a = A()
a.tmp = MagicMock(wraps=a.tmp)
b(a)
a.tmp.assert_called()

python check if a method is called without mocking it away

3 Answers3

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