verify the return value of a function

Roy Smith roy at
Fri Jan 20 09:24:04 EST 2012

In article <mailman.4872.1327005963.27778.python-list at>,
 Jabba Laci <jabba.laci at> wrote:

> Hi,
> In a unit test, I want to verify that a function returns a
> cookielib.LWPCookieJar object. What is the correct way of doing that?

jar = my_function_being_tested()
self.assertIsInstance(jar, cookielib.LWPCookieJar)

That works in 2.7.  If you're using something older than 2.7, you'll 
need to do:

self.assertTrue(isinstance(jar, cookielib.LWPCookieJar)

Alternatively, just download the 2.7 version of unittest and use that 
(it works fine with 2.6, not sure about earlier than that).
> 3) isinstance(return_value, cookielib.LWPCookieJar) seems to be the
> best way, however somewhere I read that using isinstance is
> discouraged

Where did you read that, and in what context?

Compared to type(), isinstance() is an improvement because it correctly 
handles subclasses.  If you want a LWPCookieJar, you should be happy to 
have somebody give you a subclass of LWPCookieJar (assuming they 
correctly implemented the interface).  Thus says the Church of Most 
Corpulent Staticness and Type Bondage.

On the other hand, there are some (adherents of the Most Holy and 
Loquacious Church of Duck Typing) who would say that testing for class 
at all is a sin, and what you want to do is test that the object being 
tested has the methods and attributes you expect.

Me, I'm somewhere in between.  I believe that pinching it and seeing 
what the quack sounds like is usually the right thing to do.  On the 
other hand, if you want to demand to see its Certificate of Duckiness, 
you have a right to do that too.

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