[issue27715] call-matcher breaks if a method is mocked with spec=True

[Karthikeyan Singaravelan]

Karthikeyan Singaravelan  added the comment:

I think the original issue with patch.object reported by Carl is different from 
the one reported by David for autospec. Analyzed the report by David and When 
we call autospec on a class with instance=True then the spec is modeled on the 
signature of __call__ instead of __init__ where __call__ has the signature of 
(self, x) and self is  discarded with _eat_self passed as True. But mock also 
stores _spec_signature that is not aware of skipping self and has the signature 
as (self, x) and is used for checking signature in mock.assert_called_with. 
When instance=True then kwargs['_spec_as_instance']=True so does it makes sense 
to set kwargs['_eat_self'] = True at [0] ? I applied the change and there are 
no test failures so this deserves a test.

This makes __call__ to have a different signature when it's called from mock 
and when the call is checked with assert_called_with_once. But it's not the 
case for methods of the class where self is skipped both for mock and 
_spec_signature. Since __signature__ is set for mock with my PR this makes it 
little easy to debug. Can this be dealt as a separate issue?

I would also love to see if the assertion message can be improved since 
expected_call and actual_call are printed with repr version of the call object 
in spite of the signature failure. This has caused confusion here and in other 
places like issue26752 and issue25312.

Sample program to demonstrate difference in signatures :  

import inspect
from unittest import mock

class Foo:

def __call__(self, x):
return x

def bar(self, x):
pass

m = mock.create_autospec(Foo, instance=True)
m(7)
m.bar(7)
print(inspect.signature(m))
print(m._spec_signature)
print(inspect.signature(m.bar))
print(m.bar._spec_signature)
m.bar.assert_called_once_with(7) # 7 passed as self with no value for x
m.assert_called_once_with(7) # Fails due to self

(x)
(self, x) # self is not skipped in _spec_signature
(x)
(x)
TypeError: missing a required argument: 'x'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "../backups/bpo27715_1.py", line 20, in 
m.assert_called_once_with(7)
  File 
"/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/unittest/mock.py", 
line 840, in assert_called_once_with
return self.assert_called_with(*args, **kwargs)
  File 
"/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/unittest/mock.py", 
line 827, in assert_called_with
raise AssertionError(_error_message()) from cause
AssertionError: Expected call: mock(7)
Actual call: mock(7)


[0] 
https://github.com/python/cpython/blob/f8e9bd568adf85c1e4aea1dda542a96b027797e2/Lib/unittest/mock.py#L2199

--
nosy: +cjw296, mariocj89
versions:  -Python 3.6

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[issue27715] call-matcher breaks if a method is mocked with spec=True

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


--
nosy: +xtreak
versions: +Python 3.7, Python 3.8 -Python 3.4, Python 3.5

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[issue27715] call-matcher breaks if a method is mocked with spec=True

[Serhiy Storchaka]

Change by Serhiy Storchaka :


--
type: crash -> behavior

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[issue27715] call-matcher breaks if a method is mocked with spec=True

[David Hoyes]

David Hoyes added the comment:

I came across a different failing test case, which looks a lot like the same 
issue:

```
from unittest import mock

class Foo(object):
def __call__(self, x):
return x

m = mock.create_autospec(Foo, instance=True)
m(7)
m.assert_called_once_with(7)
```

In mock 1.0.1 this passes, but in Python 3.5 we get this error:

```
TypeError: missing a required argument: 'x'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "euhpc/tmp/mockbug.py", line 12, in 
m.assert_called_once_with(7)
  File "/usr/lib/python3.5/unittest/mock.py", line 803, in 
assert_called_once_with
return self.assert_called_with(*args, **kwargs)
  File "/usr/lib/python3.5/unittest/mock.py", line 792, in assert_called_with
raise AssertionError(_error_message()) from cause
AssertionError: Expected call: mock(7)
Actual call: mock(7)
```

--
nosy: +David Hoyes

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[issue27715] call-matcher breaks if a method is mocked with spec=True

[jordan-pittier]

jordan-pittier added the comment:

I stumbled onto this today. I can confirm the issue.

--
nosy: +jordan-pittier

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[issue27715] call-matcher breaks if a method is mocked with spec=True

[Eli_B]

Changes by Eli_B :


--
nosy: +Eli_B

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[issue27715] call-matcher breaks if a method is mocked with spec=True

[Carl Meyer]

Carl Meyer added the comment:

`hg clean --all` resolved the compilation issues; confirmed that 
https://hg.python.org/cpython/rev/b888c9043566/ is at fault.

Also, the exception trace I provided above looks wrong; it must be from when I 
was messing about with `autospec=True` or passing in the instance. The actual 
trace from the sample code in the original report  has no mention of the 
instance:

```
TypeError: missing a required argument: 'x'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "../mock-method.example.py", line 11, in 
mock_bar.assert_called_once_with(7)
  File "/home/carljm/projects/python/cpython/Lib/unittest/mock.py", line 822, 
in assert_called_once_with
return self.assert_called_with(*args, **kwargs)
  File "/home/carljm/projects/python/cpython/Lib/unittest/mock.py", line 811, 
in assert_called_with
raise AssertionError(_error_message()) from cause
AssertionError: Expected call: bar(7)
Actual call: bar(7)

```

--

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[issue27715] call-matcher breaks if a method is mocked with spec=True

[Carl Meyer]

Changes by Carl Meyer :


Removed file: http://bugs.python.org/file44054/mock-method.example.py

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[issue27715] call-matcher breaks if a method is mocked with spec=True

[Carl Meyer]

Carl Meyer added the comment:

It seems likely that this regression originated with 
https://hg.python.org/cpython/rev/b888c9043566/ (can't confirm via bisection as 
the commits around that time fail to compile for me).

--
nosy: +michael.foord, pitrou

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[issue27715] call-matcher breaks if a method is mocked with spec=True

[Carl Meyer]

Carl Meyer added the comment:

(This bug is also present in Python 3.4.4.)

--
type:  -> crash
versions: +Python 3.4

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[issue27715] call-matcher breaks if a method is mocked with spec=True

[Carl Meyer]

New submission from Carl Meyer:

When constructing call-matchers to match expected vs actual calls, if 
`spec=True` was used when patching a function, mock attempts to bind the 
recorded (and expected) call args to the function signature. But if a method 
was mocked, the signature includes `self` and the recorded call args don't. 
This can easily lead to a `TypeError`:

```
from unittest.mock import patch

class Foo:
def bar(self, x):
return x

with patch.object(Foo, 'bar', spec=True) as mock_bar:
f = Foo()
f.bar(7)

mock_bar.assert_called_once_with(7)

```

The above code worked in mock 1.0, but fails in Python 3.5 and 3.6 tip with 
this error:

```
TypeError: missing a required argument: 'x'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "../mock-method.example.py", line 11, in 
mock_bar.assert_called_once_with(7)
  File "/home/carljm/projects/python/cpython/Lib/unittest/mock.py", line 203, 
in assert_called_once_with
return mock.assert_called_once_with(*args, **kwargs)
  File "/home/carljm/projects/python/cpython/Lib/unittest/mock.py", line 822, 
in assert_called_once_with
return self.assert_called_with(*args, **kwargs)
  File "/home/carljm/projects/python/cpython/Lib/unittest/mock.py", line 811, 
in assert_called_with
raise AssertionError(_error_message()) from cause
AssertionError: Expected call: bar(7)
Actual call: bar(<__main__.Foo object at 0x7fdca80b7550>, 7)
```
```

If you try to pass in the instance as an expected call arg, the error goes away 
but it just fails to match:

```
AssertionError: Expected call: bar(<__main__.Foo object at 0x7f5cbab35fd0>, 7)
Actual call: bar(7)
```

So AFAICT there is no way to successfully use `spec=True` when patching a 
method of a class.

Oddly, using `autospec=True` instead of `spec=True` _does_ record the instance 
as an argument in the recorded call args, meaning that you have to pass it in 
as an argument to e.g. `assert_called_with`. But in many (most?) cases where 
you're patching a method of a class, your test doesn't have access to the 
instance, elsewise you'd likely just patch the instance instead of the class in 
the first place.

I don't see a good reason why `autospec=True` and `spec=True` should differ in 
this way (if both options are needed, there should be a separate flag to 
control that behavior; it doesn't seem related to the documented differences 
between autospec and spec). I do think a) there needs to be some way to record 
call args to a method and assert against those call args without needing the 
instance (or resorting to manual assertions against a sliced `call_args`), and 
b) there should be some way to successfully use `spec=True` when patching a 
method of a class.

--
components: Library (Lib)
files: mock-method.example.py
messages: 272209
nosy: carljm
priority: normal
severity: normal
status: open
title: call-matcher breaks if a method is mocked with spec=True
versions: Python 3.5, Python 3.6
Added file: http://bugs.python.org/file44054/mock-method.example.py

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