Add ChaiBase.mock_of(interface) to create mock objects that have isinstance(mock, interface) = True

[tim-mitchell]

The use of isinstance checks on parameter values is quite common when using the abc module or PyContracts. The mock_of() method returns a Mock() instance that passes the isinstance test.

The interface class passed to mock_of must have a meta-class so that we can override the instancecheck method on it. This is always the case with interface classes created with the abc module or PyContracts.

[awestendorf]

@tim-mitchell looks good. please clear up the Travis errors, squash the commits and I'll merge this in

[tim-mitchell]

Hi Aaron,

Sorry, but I don’t know how to do either of those things. I have added you
as a collaborator on tim-mitchell/chai, would you mind making the
appropriate changes please.

Thanks

Tim

Tim Mitchell

Senior Software Engineer

phone: +64 3 961 1031 ext. 217 <+64-3-961-1031;ext=217>

email: tim.mitchell@leapfrog3d.com

skype: tim.mitchell.leapfrog3d

address: 41 Leslie Hills Drive, Riccarton, Christchurch 8011, New Zealand

www.leapfrog3d.com | www.aranzgeo.com

From: Aaron Westendorf [mailto:notifications@github.com]
Sent: Wednesday, 8 July 2015 6:12 a.m.
To: agoragames/chai
Cc: Tim Mitchell
Subject: Re: [chai] Add ChaiBase.mock_of(interface) to create mock
objects that have isinstance(mock, interface) = True (#28)

@tim-mitchell https://github.com/tim-mitchell looks good. please clear up
the Travis errors, squash the commits and I'll merge this in

—
Reply to this email directly or view it on GitHub
#28 (comment).

[awestendorf]

@tim-mitchell I tried every which way to push a requirements change to your master branch, kept getting errors from git complaining that I was trying to push an out-of-date branch even though I pulled down all your changes.

Speaking of which, I found several more changes in your master since this PR was opened. To keep the feature development separate from any other changes you may have, and the commit I pushed, I added the branch https://github.com/tim-mitchell/chai/tree/mock_of.

I tried using the feature on datetimes though and it didn't work. I called:

self.mock_of(datetime.datetime, obj=module_of_my_model, attr='datetime')

and saw the following two errors

    self.mock_of(datetime.datetime, obj=module_of_my_model, attr='datetime')
  File "build/bdist.linux-x86_64/egg/chai/chai.py", line 270, in mock_of
    meta_cls.__instancecheck__ = _instance_check
TypeError: can't set attributes of built-in/extension type 'type'

    super(MyTestCase, self).tearDown()
  File "build/bdist.linux-x86_64/egg/chai/chai.py", line 197, in tearDown
    setattr(mock[0], mock[1], mock[2])
TypeError: can't set attributes of built-in/extension type 'type'

On second glance through the API, I'm also wondering why we need mock_of, when we could add another kwarg parameter to mock that would suite the same purpose.

[tim-mitchell]

@awestendorf The changes since the pull request are fixes to get mock_of to work correctly on python 3. I am not doing anything else on this fork. I could not get travis to work for me to test on pypy.

mock_of() is intended to work with interfaces created with the abc module, not with types such as datetime. I don't know that you can override the isinstance check of types in general. Still, a better error message would be good in this situation.

No particular reason for separate mock_of method, am happy to fold it into mock if you would prefer.