Required Reading: "How Non-Member Functions Improve Encapsulation"

[w0rp]

I've noticed the benefits of writing non member functions in 
Python codebases. Say if you have a User model in a Django ORM, 
and you have a Thing model, and some operation on User and Thing. 
I've noticed that your code is almost always better if you write 
a non member function on User and Thing, instead of a member of 
User or Thing.

Often a function belongs to neither type. Instead the logic cuts 
across those two types. The key disadvantage I notice is ending 
up with very large and unreadable classes which poorly categorise 
business logic, when you could have been categorising functions 
in modules based on different business needs.