copy.copy(object) returns an object with id == None during tests?

I am new to Django and Python, building my first app using TDD... I wanted to copy an instance of Task, a model object. I used the following code, which works correctly during the tests:

import copy

class Task(models.Model): 
    ...
    def make_copy(self):
       new_task = copy.copy(self)
       new_task.save()
       return new_task

But when running this code 'normally开发者_如何转开发', in the server, I noticed it was not working: no new object was created. I found out I had to add new_task.id = None just before saving, and I understand the reason for this...

But if copy.copy does not know about Django and thus will not change the id itself, why is it the case that the object returned has id == None during the tests?

It sounds like your test case is not fully matching the usage in your "normal" use case.

The id field is set for objects that exist in the database. If you pass your make_copy() method an object with id set, it will appear to fail because it's not making a new database object, it's just saving an existing object (through the Python copy, with an existing id).

I'd guess your test case is passing Task objects to make_copy() with id of None (and therefore appearing to work), while in "normal" usage, the objects are coming in with id set. (You could test this hypothesis with a simple print or assert statement).

One simple solution might be to set id to None right after your copy operation. That way, a new database object is always created.

Finally, someone else with the same situation: http://www.nerdydork.com/copy-model-object-in-django.html