How to reinitialize a method within python class with updated object value

Question

I want to have a class and be able to change an object value and have it flow through to methods and objects created in the class. In the below example I am trying have the new value 3 flow through but I keep getting the original value 2. Am I doing it wrong?

class fun:
  g=2
  def a(c=g):
    return c
  z=g

fun.g = 3
fun.a()
fun.z

I have searched for __init__ functions but I don't see how they can help here. I tried created a new class making is the same as the fun class. That didn't work either.

You're facing a couple different issues here, but It sounds like you're seeking getters and setters. In python the @property decorator is used for this purpose. — Teejay Bruno
– Teejay Bruno, Commented Jul 20, 2021 at 15:44
Changing the default argument value of an existing function is possible, though not really recommended. — chepner
– chepner, Commented Jul 20, 2021 at 15:50
Thanks for the info and reading material, super helpful. I have to approach in totally different way — EoinS
– EoinS, Commented Jul 20, 2021 at 16:01

Epsi95 · Accepted Answer · 2021-07-20 15:50:38Z

3

as otheres mentioned, just want to point out that def a(c=g): the g has been replaced with 2 during class creation.

so after

fun.g = 3

print(fun.__dict__)

mappingproxy({'__module__': '__main__',
              'g': 3,
              'a': <function __main__.fun.a(c=2)>,
              'z': 2,
              '__dict__': <attribute '__dict__' of 'fun' objects>,
              '__weakref__': <attribute '__weakref__' of 'fun' objects>,
              '__doc__': None})

As you can see 'a': <function __main__.fun.a(c=2)>,

answered Jul 20, 2021 at 15:50

Epsi95

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quamrana · Accepted Answer · 2021-07-20 15:48:44Z

2

In python, methods and functions have their default values calculated and stored when the method or function is declared.

You can defer that calculation by doing it inside the method at runtime:

class fun:
  g=2
  def a(c=None):  # None is used here as a sentinal value
    if c is None:
        c=fun.g
    return c
  z=g

answered Jul 20, 2021 at 15:48

quamrana

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4 Comments

chepner Over a year ago

This doesn't update the value of fun.z, though. I think the idea was to be able to update several things depending on the value of fun.g with only one change.

chepner Over a year ago

z = property(lambda self: self.g), perhaps.

quamrana Over a year ago

Well, just omit z. Use g instead!

EoinS Over a year ago

@chepner you are right, really just shorthand sample for how to make several updates with one change...unfortunately I can't seem to find a way to do this in a way that makes the code simple/readable. Essentially tons of methods/objects within a class need to inherit this value which will change from use to use

S.B · Accepted Answer · 2021-07-20 15:54:57Z

The problem is that parameters of a function is evaluated at compile time not runtime! Because in compile time the value of g is 2 and in your function you return that value, it's always going to be 2.

As a point, you call a() function on the class itself, so it's first parameter is not going to be filled with a reference to an instance. Story changes when you try to call it from instance of that class.

For demonstration take a look at this :

from datetime import datetime
from time import sleep

class C:
    def fn(self, date=datetime.now()):
        return date

obj = C()
print(obj.fn())
sleep(2)
print(obj.fn())

They both have same value.

The solution is to put g equal to None for default value and then inside the function check whether that value is passed or not:

from datetime import datetime
from time import sleep

class C:
    def fn(self, date=None):
        if date:
            return date
        return datetime.now()

obj = C()
print(obj.fn())
sleep(2)
print(obj.fn())

Amos Baker · Accepted Answer · 2021-07-20 17:28:53Z

1

Not sure quite what you mean by reinitialize, __init__ occurs only on creation. If you create a new fun object it will be initialized in the same way with the same value.

$ cat fun.py

class fun(object):

    def __init__(self):
        self.g = 2

    def a(self, c=None):
        if c is None:
            c = self.g
        return c


first = fun()
second = fun()

first.g = 3

print(first.g)
print(first.a())
print(second.g)
print(second.a())

$ python3 fun.py
3
3
2
2

answered Jul 20, 2021 at 17:28

Amos Baker

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