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Python Object-Oriented Python Advanced Objects Emulating Built-ins

Posted by Benyamin Kohanchi
Benyamin Kohanchi
Courses Plus Student 2,342 Points

The difference between dunder len and len()

Kinda confused between dunder len and len(), is dunder len object-oriented while len() is not?

Thanks in advance 😁

1 Answer

Josh Keenan
Josh Keenan
20,315 Points
Josh Keenan
Josh Keenan
20,315 Points

Well __len__ is len(). When you call the function, it uses that objects __len__ method to evaluate what it should do with this type of object. It was thought that it would be easier using len(a) instead of a.len(), so they got it to work like this. Sorry I don't know as much about this as I should, but I hope this helps!

Benyamin Kohanchi
Benyamin Kohanchi
Courses Plus Student 2,342 Points

Great, thanks Josh!

Chris Freeman
Chris Freeman
Treehouse Moderator 68,441 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,441 Points

Great answer. Most of the dunder methods are defined to be called by a corresponding built-in function. If one really wanted to have a.len() functionality also work, a len method could be added (though not recommended):

def a:
    def len(self):  # len method called using a.len()
        return len(self)  # call to built-in len() function

So, a.len() calls the len method that, in turn, calls the built-in function len() which would call the __len__ method. The __len__ method might be defined locally or inherited from a standard type. Caution: the line return self.len(self) would cause a recursive stack overflow since the method calls itself. Good luck!!

Benyamin Kohanchi
Benyamin Kohanchi
Courses Plus Student 2,342 Points

Thanks for the example Chris, it helped!