python object to accept len function to run against it

Question

I have a python object which stores details of countries, what method should I implement to allow running the object into len(ObjectInstance)?

For example, I want to count all the countries in the instance and I must use len() the function because its a third party call/external.

How can I implement a method that would be called when len() is executed against the instance?

Answer 1

Implement __len__() magic method:

>>> class A():
...     def __len__(self):
...         return 100
... 
>>> a = A()
>>> len(a)
100

Also see:

• Making Custom Sequences paragraph of "A Guide to Python's Magic Methods"
• Why does python use 'magic methods'?
• difference between len() and .__len__()?