Is writing data members inside ctor more pythonic?

Question

My limited experience in python shows that

class Person:
    def __init__(self):
        self.data: Tuple[int, str] = (5, "M")
        print("DONE")

is more common than

class Person:
    data: Tuple[int, str] = (5, "M")
    def __init__(self):
        print("DONE")

Any reason to favor the first version?

The _behaviour_ is fundamentally different, although it's less relevant with immutable attributes like tuples; the former creates an instance attribute, the latter a class attribute. — jonrsharpe, Oct 24 '21 at 11:42

score 2 · Accepted Answer · answered Oct 24 '21 at 11:50

This is creating different attributes, the first one is an instance attribute (unique to each instance of the object), the second a class attribute, that is shared among the objects of this class.

Here is a quick demo (I changed the attribute to a mutable object):

from typing import List, Union

class PersonA:
    def __init__(self):
        self.data: List[Union[int, str]] = [5, "M"]
        print("DONE")
        
class PersonB:
    data: List[Union[int, str]] = [5, "M"]
    def __init__(self):
        print("DONE")

pa1 = PersonA()
pa2 = PersonA()
pb1 = PersonB()
pb2 = PersonB()

Now let's mutate one object and looks at the other:

>>> pa1.data[0] = 1
>>> pa2.data
[5, 'M']  # unchanged, as instance attribute

>>> pb1.data[0] = 1
>>> pb2.data
[1, 'M']  # changed as affects all objects of the class

Is writing data members inside ctor more pythonic?

1 Answers1