Max recursion depth while trying to mock instance method

Question

I want to mock an instance method. First I want to modify the arguments and then I want to call the original method.

I tried this:

import mock

class Foo(object):
    def my_method(data):
        print(data)

def wrapped_method(data):
    return Foo.my_method(data.replace('x', 'o'))

with mock.patch.object(Foo, 'my_method', wraps=wrapped_method):
    foo = Foo()
    foo.my_method('axe')  # should print "aoe"

But I get this exception:

/home/foo/bin/python /home/foo/src/wrap-instance-method.py
Traceback (most recent call last):
  File "/home/foo/src/wrap-instance-method.py", line 15, in <module>
    foo.my_method('axe')  # should print "aoe"
  File "/home/foo/local/lib/python2.7/site-packages/mock.py", line 955, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/home/foo/local/lib/python2.7/site-packages/mock.py", line 1024, in _mock_call
    return self._mock_wraps(*args, **kwargs)
.....
    return self._mock_wraps(*args, **kwargs)
  File "/home/foo/src/wrap-instance-method.py", line 10, in wrapped_method
    return Foo.my_method(data.replace('x', 'o'))
  File "/home/foo/local/lib/python2.7/site-packages/mock.py", line 955, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/home/foo/local/lib/python2.7/site-packages/mock.py", line 960, in _mock_call
    self.called = True
RuntimeError: maximum recursion depth exceeded while calling a Python object

Process finished with exit code 1

How can I call the original method which was wrapped without recursion exception?

Could you please tell me why you down-vote this question?

guettli
– guettli

2017-08-09 08:01:33 +00:00
Commented Aug 9, 2017 at 8:01 — guettli
– guettli, Commented Aug 9, 2017 at 8:01

Martijn Pieters · Accepted Answer · 2017-08-09 10:09:16Z

14

You replaced the Foo.my_method with a method that calls Foo.my_method, so yes, you'll get an infinite recursion.

If you must have the original method, you need to store it before you patch it:

def mock_method_factory(original):
    def wrapped_method(data):
        return original(data.replace('x', 'o'))
    return wrapped_method

with mock.patch.object(Foo, 'my_method', wraps=mock_method_factory(Foo.my_method)):
    # ...

However, wraps does not handle binding; original is an unbound function and wrapped_method won't be passed in self.

The mock library is perhaps not the best choice here. What you are essentially doing is apply a temporary decorator. Either subclass Foo or apply the decorator manually to Foo.my_method:

def mock_method_factory(original):
    def wrapped_method(self, data):
        return original(self, data.replace('x', 'o'))
    return wrapped_method

class MockedFoo(Foo):
    my_method = mock_method_factory(Foo.my_method)

foo = MockedFoo()
foo.my_method('axe')  # should print "aoe"

or

def mock_method_factory(original):
    def wrapped_method(self, data):
        return original(self, data.replace('x', 'o'))
    return wrapped_method

original = Foo.my_method
try:
    Foo.my_method = mock_method_factory(original)
    foo = Foo()
    foo.my_method('axe')  # should print "aoe"
finally:
    Foo.my_method = original

edited Aug 9, 2017 at 10:09

answered Aug 9, 2017 at 7:54

Martijn Pieters

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7 Comments

guettli Over a year ago

I added "return wrappend_method" since I think it is needed. Now I get "TypeError: unbound method my_method() must be called with Foo instance as first argument (got str instance instead)"

Martijn Pieters Over a year ago

@guettli: ah, yes, the replacement callable is not a method, so it won't be bound and no self is passed in.

guettli Over a year ago

Yes, not self is passed in. Sad. Is there an other way? I just want to do some kind of overwrite an instance method (comparable to inheritance and calling the parent method in the child method).

Martijn Pieters Over a year ago

@guettli: added other ways that don't use unittest.mock.

guettli Over a year ago

Yes, the second solution (try..except (without mock)) works. I just need to add "self" here: def wrapped_method(self, data)

|

Stephanie Rong · Accepted Answer · 2021-07-07 05:18:01Z

0

I encountered the same issue. Tried with Martijn's solution, but had the self problem. Solved by the following:

real_constructor = CLASS_NAME.__init__
def mock_constructor(self, **args):
    return real_constructor.(self=self, **args)

with mock.patch.object(Foo, '__init__', side_effect=mock_constructor):
    ...

answered Jul 7, 2021 at 5:18

Stephanie Rong

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