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On 29/07/2011 18:03, Tom Davis wrote:
<blockquote cite="mid:4E32E7EA.8090209@recursivedream.com"
type="cite">
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On 07/29/2011 12:42 PM, Michael Foord wrote:
<blockquote cite="mid:4E32E2E7.1030804@voidspace.org.uk"
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>
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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<br>
<br>
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</blockquote>
<br>
<br>
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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 <a class="moz-txt-link-freetext" href="http://www.sqlite.org/different.html">http://www.sqlite.org/different.html</a>
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