Question

So I have a class like this:

 class Encoder(object):

     movie = None

     def __init__(self, movie_instance):
         self.movie = movie_instance

     def encode_profile_hd(self):
         print "profile 1"

     def encode_profile_sd(self):
         print "profile 2"

How can I specify the movie_instance argument passed to the constructor must be of Movie class?

I tried:

def __init__(self, Movie movie_instance):

But that doesn’t work. Sorry if this is obvious.

Answer 1

you can use isinstance for that.

def __init__(self,movie_instance):
    if(not isinstance(movie_instance, Movie)):
         raise ValueError("movie_instance must be an instance of class Movie")

This will work for Movie instances as well as anything that inherits from Movie.

It’s worth noting that a lot of people would say that in Python, this idiom should not be used often. If something looks like a movie, smells like a movie (do movies have a scent?) and plays like a movie, why not just treat it like a movie? (In other words, try to use the object like a movie and handle the exception that is raised if it doesn’t work).