I have a python app which can load and run extensions. It looks like this:

import importlib, sys

sys.path.append('.')

module = importlib.import_module( 'extension' )
Extension = getattr(module, 'Extension')
instance = Extension()

instance.FunctionCall(arg1='foo')

The users can create their own loadable extension by implementing a class I defined. In this case, it is in extension.py:

class Extension:
    def FunctionCall(self, arg1):
        print(arg1)

This works great for now, but I would like to extend my interface to pass another argument into Extension.FunctionCall

instance.FunctionCall(arg1='foo', arg2='bar')

How can I do this while maintaining reverse compatibility with the existing interface?

---

Here's what happens to existing users if I make the change:

$ python3 loader.py 
Traceback (most recent call last):
  File "loader.py", line 9, in <module>
    instance.FunctionCall(arg1='foo', arg2='bar')
TypeError: FunctionCall() got an unexpected keyword argument 'arg2'

---

One option is to catch the TypeError, but is this the best solution? It looks like it could get unmaintainable with several iterations of interfaces.

try:
    instance.FunctionCall(arg1='foo', arg2='bar')
except TypeError:
    instance.FunctionCall(arg1='foo')

Another option is to define the "interface version" as a property of the extension and then use that in a case statement, but it would be nicer to simply detect what is supported instead of pushing overhead to the users.

if hasattr(Extension, 'Standard') and callable(getattr(Extension, 'Standard')):
    standard=instance.Standard()
else:
    standard=0

if standard == 0:
    instance.FunctionCall(arg1='foo')
elif standard == 1:
    instance.FunctionCall(arg1='foo', arg2='bar')