Method vs function-valued field in Python

Question

I am switching from MATLAB to Python and numpy and I would like to know if there is any difference between the option to define a class method and the option to the function to a class field (instance variable)? Here is the example:

class MyClass:
    def __init__(self, a):            
        self.a=a    #some variable

    def add(self,b):
        return self.a+b

vs

class MyClass:
    def __init__(self, a):
        self.a=a    #some variable            
        self.add = lambda b: self.a+b

It works in both cases when I call

my_object=MyClass(2)
print(my_object.add(2)) #prints 4

Are there any differences between these two approaches? Any best practices/downsides? To me, the first one feels more "proper OOP", but the second one feels more flexible. Or, maybe, the definitions are identical, because of the way Python works under the hood?

The first one is a method, the second one is just a function that is an attribute. Note, for a method, a single function object on the class, whereas if you assign a function to an attribute, a separate function object exists for every instance. — juanpa.arrivillaga, Jun 19 '20 at 20:12
Best practice is not to use a `lambda` function and assign it to a variable, see here: https://stackoverflow.com/questions/25010167/e731-do-not-assign-a-lambda-expression-use-a-def. — Dr. V, Jun 19 '20 at 20:12

score 2 · Answer 1 · answered Jun 19 '20 at 20:11

The second one can't be overridden and takes a lot more space, because there's a separate function in every instance's __dict__ instead of one function in the class __dict__. (Instance method objects are created and reclaimed on the fly if you do it the normal way, or optimized out entirely in many cases depending on Python version.)

Method vs function-valued field in Python

1 Answers1