Ensure the user chain the tool in a proper order

[zhukovgreen]

Currently nothing forbids to do when(...).then_return, but
it is expected only when.called_with.then_return

An exception should be raised in inproper order of the methods

[absalukaskosina]

I was thinking about that when I was testing how to do no args.

What do you think that instead of raising exception, using when would return object that simply does not have .then_return function?

I know it would take some code refactoring, but it might be easier for users to understand, which methods should be used in each step.

Something like:

class Z:
      ....

class Y:
    def then_return(signature) -> Z: 
      ....

class X: 
   def called_with(signature) -> Y:
      ...

def when(signature) -> X:
   .....

So when you call when(...) IDE would simply not whisper you to use then_return, because X does not have it there.