classification
Title: Mock calls on mutable objects
Type: behavior Stage: resolved
Components: Tests Versions: Python 3.6, Python 3.5
process
Status: closed Resolution: duplicate
Dependencies: Superseder: Python unittest.mock.mock_calls stores references to arguments instead of their values
View: 28318
Assigned To: Nosy List: mcscope@gmail.com, pawelj
Priority: normal Keywords:

Created on 2018-05-28 13:12 by pawelj, last changed 2018-05-28 20:11 by r.david.murray. This issue is now closed.

Files
File name Uploaded Description Edit
test.py pawelj, 2018-05-28 13:12
unintuitive_side_effect.py mcscope@gmail.com, 2018-05-28 16:32
Messages (3)
msg317849 - (view) Author: Pawel (pawelj) Date: 2018-05-28 13:12
When method of mocked object is called multiple times with mutable object as parameter and attribute of this object has been updated between calls, mock_calls is only aware of the last value. I think it unexpected behaviour, that mock doesn't track value changes.
I've attached example code
msg317884 - (view) Author: Lady Red (mcscope@gmail.com) * Date: 2018-05-28 16:32
To have the behavior that you are expecting, I believe the mock would have to store a deep copy of every mutable object that is passed in to any of it's calls, in case that object later mutates. 

That would really expand the memory and time footprint of working with a Mock object.  The mock object would grow with every call to. (This is already true, as it's adding to it's mock_calls list... but it would grow much more. )

I'm also not sure about the safety of trying to deepcopy all objects passed in. 

I think that in addition to the safety and memory/performance issues that I've highlighted, deepcopying all objects would lead to it's own unintuitive behaviors.  Objects you get back from mock_call wouldn't be the objects that it was actually called with, they would be copies. 

I am attaching a file that illustrates this last point.
msg317902 - (view) Author: Pawel (pawelj) Date: 2018-05-28 19:28
Ok, fair enough. Thank You for explanation. I was just surprised with this behaviour.
On the other hand, I don't like mutable objects and I'm trying to avoid them.
History
Date User Action Args
2018-05-28 20:11:38r.david.murraysetstatus: open -> closed
superseder: Python unittest.mock.mock_calls stores references to arguments instead of their values
resolution: duplicate
stage: resolved
2018-05-28 19:28:47paweljsetmessages: + msg317902
2018-05-28 16:32:35mcscope@gmail.comsetfiles: + unintuitive_side_effect.py
nosy: + mcscope@gmail.com
messages: + msg317884

2018-05-28 13:12:45paweljcreate