verify the return value of a function

Ulrich Eckhardt ulrich.eckhardt at
Fri Jan 20 04:15:54 EST 2012

Am 19.01.2012 21:45, schrieb Jabba Laci:
> In a unit test, I want to verify that a function returns a
> cookielib.LWPCookieJar object. What is the correct way of doing that?
> 1) First I tried to figure out its type with type(return_value) but it
> is<type 'instance'>

I'm not sure where the problem here is and where exactly you are seeing 
this. This might even indicate a problem with how the returned type is 


 >>> x = 1
 >>> type(x)
<type 'int'>
 >>> type(x) is int

So checking for an exact type should work using type().

> 2) return_value.__class__ .__name__ gives 'LWPCookieJar', which is bettter

It doesn't cover namespaces though. Also, you should compare that to 
cookielib.LWPCookieJar.__name__, not 'LWPCookieJar'. What is the "LWP", btw?

> 3) isinstance(return_value, cookielib.LWPCookieJar) seems to be the
> best way, however somewhere I read that using isinstance is
> discouraged.

Never trust any such claim that doesn't give a justification. In your 
case, that would be the right thing to do, IMHO. Promising to return an 
LWPCookieJar is fulfilled when the returnvalue is of that type or a 
class derived from that, which variant 1 doesn't cover.


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