There is a (relatively) widely used technique in unit testing, called
mock objects. There is even a pMock library which provides a Mock class
for a Python environment.
Given the "duck typing" nature of the Python itself, it's pretty
trivial to build mocks without using any pre-built libraries. What is
less trivial and potentially more worthwhile is to have a library of
"stock" mock objects.
For instance, I found myself re-implementing ConnectionStub class again
and again for different projects. And there is no shortage of other
good candidates as well: socket, web request/response, thread objects,
etc. Someone on a Python mailing list asks whether anyone implemented a
mock filesystem interface, for example.
Having a library of such mock classes, realizing widely-used and
"heavy" interfaces may be a good idea.
The only real problem I can think of is whether it's possible to make
these mocks generic enough. Mocks often contain some
(application-specific) hard-coded rules and data to realize desired
behavior. For instance, socket mock may fail when user attempts to
write a "foobar" string - and unit test will use this to check
how code handle these kind of errors. These rules may be generalized
but it may lead to mock classes becoming unwieldy.
Even keeping this problem in mind, it's still a reasonable idea for a
library, but obviously I'm biased.
What do you think?