isA function?

[Roy Smith]

Is there a standard isA() function in Python?  It should take two 
arguments, an instance and a class, and return true if the object is a 
member of the class or one of the class's ancestors.

I know I can roll this myself with __class__.__bases__, but it seems 
like a common enough thing that there would be a built-in way to do it.  
I looked at the inspect module, and was surprised to not find it there.

We're writing some unit tests, and one of the tests is "foo() returns 
something which isA (bar)".