[TIP] Issue with "mock" package and isinstance()

[Michael Foord]

On 29/07/2011 18:37, Tom Davis wrote:
> 
> On 07/29/2011 01:29 PM, Michael Foord wrote:
>> 
>> On 29/07/2011 18:03, Tom Davis wrote:
>>> 
>>> On 07/29/2011 12:42 PM, Michael Foord wrote:
>>>> 
>>>> On 29/07/2011 17:31, Tom Davis wrote:
>>>>> 
>>>>> Hey all,
>>>>> 
>>>>> I've been using Michael Foord's awesome mock package and I've loved it... now I'm running into an issue. I'm patching a class, pretty simple:
>>>>> from my.module.path import Class
>>>>> patch = mock.patch('my.module.path.Class', spec=Class)
>>>>> mocked = patch.start()
>>>>> mocked.return_value = MyReplacementClass()
>>>>> I'm doing things this way because I need to mock out a class, but provide an alternate implementation that uses similar logic to the real class (basically mocking an external API by turning it into a memory-mapped API).
>>>>> 
>>>>> Somewhere else, my code gets this mock client and a function calls:
>>>>> isinstance(client, my.module.path.Class): ...
>>>>> This call fails with:
>>>>> TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and type
>>>>> This was confusing to me, so I did a bit of digging to figure out what Class really was, but all was well. Here's what I got:
>>>>> <MagicMock spec='Class' id='50389456'>
>>>> 
>>>> Well yes - what you have here is a mock *instance* - it isn't a class it's a mock of a class. So you can't use this as the second argument to isinstance. This is one of the problems with type checking (not that it's always wrong - just that it causes this kind of difficulty).
>>>> 
>>>> Actually it *would* be possible to make a mock object behave like a class, by providing __bases__ = (type,) on mock instances (or whatever the appropriate bases would be).
>>> First off, thanks for the super fast reply! I'm trying to wrap up this project and the intricacies of the final patching           stage are confusing me a bit. So if I understand this correctly, you're saying that isinstance() is actually calling (or otherwise obtaining what i set as "return_value") the mock?
>> 
>> Not quite, no. 
>> 
>> In your test you are replacing "my.module.path.Class" with a mock. So when your code tries to use "my.module.path.Class" as the second argument to isinstance, it finds the mock. This is what your patching has done. You are telling patch to replace Class with a mock - so that is what has happened! Unless I have misunderstood - but that's what it looks like from both your code and description...
>> 
> Quite right! However--and maybe this is where I am confused--I was under the impression that providing the "spec" or "spec_set" argument to Mock (or in this case, indirectly to Mock via patch()) allowed it to pass isinstance() tests.
> 

Right - a mock created with a spec of Class will pass an isinstance check for Class (the first argument to isinstance). That *does not* mean that a mock with a spec of Class can be used as the second argument.

> What I *really* want is for calling Class() to return a pre-built replacement. I don't want Class itself to be an instance of anything, if I can help it. I want it to be a class that "looks" just like the original, except for the fact that it returns (e.g. via __new__) a custom subclass. I thought patching it and providing "return_value" on the patched result would do this. But I guess I am creating Mock instances instead? Maybe my "mocked" object just needs to be a sublass of Class with __new__ replaced? Perhaps via the "new" argument to patch()? I will continue to investigate.

Well, patching Class.__new__ with a mock that returns a mock with a spec of Class should work. 

Michael


>> Michael
>> 
>>> The result would make sense, then, though I thought it just checked its type? This is a bit above my meta-Python experience.
>>>> 
>>>> Can you add this as a feature request:
>>>> 
>>>>     https://code.google.com/p/mock/issues/list
>>>> 
>>> Provided I actually understand what it is (see above), I certainly will.
>>> 
>>> Thanks again!
>>> 
>>> -Tom
>>>> All the best,
>>>> 
>>>> Michael Foord
>>>>> Seems right to me! Additionally, if I change the instance check in the production code to:
>>>>> isinstance(client, my.module.path.Class._spec_class): ...
>>>>> It works just fine! I can't do this for obvious reasons, but it seems to prove that the MagicMock at least has the correct information somewhere, it's just not being properly inspected by isinstance().
>>>>> 
>>>>> Any thoughts? I really need this to work.
>>>>> 
>>>>> 
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>>>> 
>>>> 
>>>> -- 
>>>> http://www.voidspace.org.uk/
>>>> 
>>>> May you do good and not evil
>>>> May you find forgiveness for yourself and forgive others
>>>> May you share freely, never taking more than you give.
>>>> -- the sqlite blessing http://www.sqlite.org/different.html
>>> 
>> 
>> 
>> -- 
>> http://www.voidspace.org.uk/
>> 
>> May you do good and not evil
>> May you find forgiveness for yourself and forgive others
>> May you share freely, never taking more than you give.
>> -- the sqlite blessing http://www.sqlite.org/different.html
> 


-- 
http://www.voidspace.org.uk/

May you do good and not evil
May you find forgiveness for yourself and forgive others
May you share freely, never taking more than you give.
-- the sqlite blessing http://www.sqlite.org/different.html
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