Add patch() functionality

[Jonathan Lange]

Twisted has a patch() method on TestCase that lets you easily monkey-patch existing attributes on objects. This would be very nice to have in testtools.

[Robert Collins (lifeless)]

Jono; this seems like something we shouldn't have in testtools. Rather:
There should be a library for monkey patching.
It should have an API we can trivially use (e.g. it returns (callback, (args), {kwargs}), which users can pass to addCleanup:

def newfoo():
    pass
self.addCleanup(monkey.patch(sys.time, newfoo))

What do you think?

[Jonathan Lange (jml)]

I don't feel strongly about it.

'TestCase.patch' has proven itself to be very convenient and readable in Twisted.
    self.patch(sys, 'time', newfoo)
is more readable than:
    self.addCleanup(monkey.patch, sys, 'time', newfoo)
or even
    self.addCleanup(monkey.patch(sys.time, newfoo))

And if monkey.patch can be implemented in the last way (I'm not so sure), then TestCase.patch could be written as:
    self.patch(sys.time, newfoo)

Which makes the intent very clear.

[Jonathan Lange (jml)]

I've changed my mind. I *do* feel strongly about it. This is such a useful, basic thing that it should be included in testtools.