[issue42556] unittest.mock.patch() cannot properly mock methods

[Mario Corchero]

Mario Corchero  added the comment:

Right, I believe this is indeed broken.

This code:
```
class A:
def m(self, a=None): pass

from unittest import mock
with mock.patch.object(A, "m", autospec=True):
A.m.side_effect = print
A().m(1)
```

prints: <__main__.A object at 0x7f69cc7dc6a0> 1

Whilst the same code without autospec:
```
class A:
def m(self, a=None): pass

from unittest import mock
with mock.patch.object(A, "m"):
A.m.side_effect = print
A().m(1)
```

prints: 1

This is a rather tricky issue though, as changing the behaviour to pass self 
would break too many people (even if it would probably the be "right" fix).

You can indeed use autospec or just write a descriptor that does the proper 
bound method logic:

```
class A:
def m(self, a=None): pass

def descriptor(self, obj, objtype=None):
self._self = obj
def _(*args):
return self(obj, *args)
return _

from unittest import mock
with mock.patch.object(A, "m"):
A.m.side_effect = print
A.m.__get__ = descriptor
A().m(1)
```

When patching a method at the class level, today Mock is basically hiding 
"self" when used without a spec. We should decide whether:
- that is OK and we want to "fix" autospec to do the same (remove self)
- Leave things as they are, even if not perfect
- Pass self, breaking most assertions across the world (sounds like a bad idea 
initially)
- Create some opt-in parameter or a custom thing like PropertyMock to pass self 
(MethodMock?).

TBH, I don't have a good answer :/. I'm subscribing other more insightful 
people to the issue, maybe this is the way it is intended to work :).

--
nosy: +cjw296, mariocj89, michael.foord, xtreak

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue42556] unittest.mock.patch() cannot properly mock methods

[Pierre Ossman]

New submission from Pierre Ossman :

unittest.mock.patch() as it currently works cannot properly mock a method as it 
currently replaces it with something more mimicking a function. I.e. the 
descriptor magic that includes "self" isn't properly set up.

In most cases this doesn't really matter, but there are a few use cases where 
this is important:

1. Calling base classes where you need to make sure it works regardless of 
super() or direct reference to the base class.

2. Multiple objects calling the same base class using super(). Without the self 
argument you can't tell the calls apart.

3. Setting up a side_effect that needs access to the object. In some cases you 
can pass the object using some side channel, but not all. E.g. not when mocking 
a base class' __init__(). (already reported as Issue35577).

Right now you can work around this by using autospec, as that has the 
undocumented side-effect of properly setting up methods. So don't fix 
Issue41915 before this one or we lose that workaround. :)

--
components: Library (Lib)
messages: 382415
nosy: CendioOssman
priority: normal
severity: normal
status: open
title: unittest.mock.patch() cannot properly mock methods
versions: Python 3.10, Python 3.6, Python 3.7, Python 3.8, Python 3.9

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com