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On 07/29/2011 01:29 PM, Michael Foord wrote:
<blockquote cite="mid:4E32EE16.9010509@voidspace.org.uk" type="cite">
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On 29/07/2011 18:03, Tom Davis wrote:
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type="cite">
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On 07/29/2011 12:42 PM, Michael Foord wrote:
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type="cite">
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On 29/07/2011 17:31, Tom Davis wrote:
<blockquote cite="mid:4E32E070.3060400@recursivedream.com"
type="cite">
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Hey all,<br>
<br>
I've been using Michael Foord's awesome <i>mock</i> package
and I've loved it... now I'm running into an issue. I'm
patching a class, pretty simple:<br>
<blockquote>from my.module.path import Class<br>
patch = mock.patch('my.module.path.Class', spec=Class)<br>
mocked = patch.start()<br>
mocked.return_value = MyReplacementClass()<br>
</blockquote>
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).<br>
<br>
Somewhere else, my code gets this mock client and a function
calls:<br>
<blockquote>isinstance(client, my.module.path.Class): ...<br>
</blockquote>
This call fails with:<br>
<blockquote>TypeError: isinstance() arg 2 must be a class,
type, or tuple of classes and type<br>
</blockquote>
This was confusing to me, so I did a bit of digging to
figure out what <i>Class</i> really was, but all was well.
Here's what I got:<br>
<blockquote><MagicMock spec='Class' id='50389456'><br>
</blockquote>
</blockquote>
<br>
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).<br>
<br>
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).<br>
</blockquote>
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? </blockquote>
<br>
Not quite, no. <br>
<br>
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...<br>
<br>
</blockquote>
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.<br>
<br>
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.<br>
<blockquote cite="mid:4E32EE16.9010509@voidspace.org.uk" type="cite">
Michael<br>
<br>
<blockquote cite="mid:4E32E7EA.8090209@recursivedream.com"
type="cite">The result would make sense, then, though I thought
it just checked its type? This is a bit above my meta-Python
experience.<br>
<blockquote cite="mid:4E32E2E7.1030804@voidspace.org.uk"
type="cite"> <br>
Can you add this as a feature request:<br>
<br>
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="https://code.google.com/p/mock/issues/list">https://code.google.com/p/mock/issues/list</a><br>
<br>
</blockquote>
Provided I actually understand what it is (see above), I
certainly will.<br>
<br>
Thanks again!<br>
<br>
-Tom<br>
<blockquote cite="mid:4E32E2E7.1030804@voidspace.org.uk"
type="cite"> All the best,<br>
<br>
Michael Foord<br>
<blockquote cite="mid:4E32E070.3060400@recursivedream.com"
type="cite">
<blockquote> </blockquote>
Seems right to me! Additionally, if I change the instance
check in the production code to:<br>
<blockquote>isinstance(client,
my.module.path.Class._spec_class): ...<br>
</blockquote>
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().<br>
<br>
Any thoughts? I really need this to work.<br>
<br>
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May you find forgiveness for yourself and forgive others
May you share freely, never taking more than you give.
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