In python is there a way to know if an object "implements an interface" before I pass it to a function?

Question

I know this may sound like a stupid question, especially to someone who knows python's nature, but I was just wondering, is there a way to know if an object "implements an interface" so as to say?

To give an example of what I want to say:

let's say I have this function:

def get_counts(sequence):
     counts = {}
     for x in sequence:
         if x in counts:
             counts[x] += 1
         else:
             counts[x] = 1
     return counts

My question is: Is there a way to make sure that the object passed to the function is iterable? I know that in Java or C# I could do this by having the method accept any object that implements a specific interface, let's say (for example) iIterable like this: void get_counts(iIterable sequence)

My guess is that in Python I would have to employ preemptive introspection checks (in a decorator perhaps?) and throw a custom exception if the object doesn't have an __iter__ attribute). But is there a more pythonic way to do this?

Answer 1

Python (since 2.6) has abstract base classes (aka virtual interfaces), which are more flexible than Java or C# interfaces. To check whether an object is iterable, use collections.Iterable:

if isinstance(obj, collections.Iterable):
    ...

However, if your else block would just raise an exception, then the most Python answer is: don't check! It's up to your caller to pass in an appropriate type; you just need to document that you're expecting an iterable object.