output values change

[akbarza]

hi
int the below code:

def something(num,my_list):
    num=3
    my_list[0]=3

a=2
b=[2]
something(a,b)
print(a)
# 2 is printed
print(b)
#[3] is printed

the value of a in the final is the same as before, but the b value has changed. Why?
I read somewhere something before it, but i can not understand it. can you explain it?
thanks

[deanhystad]

a and b are just names used to reference objects. The names are of no importance here.

a references an int object. Int objects are immutable, they cannot change. 2 is always 2, and you cannot change 2 to 3. You can reassign a variable to reference a different int object. num first referenced 2, then was assigned to reference 3. The only relationship between a and num is that the int object referred to by a was passed to a function as the argument num. The variables a and num know nothing about each other. Even if you rewrote the function to look like below.

def something(a,b):
    a=3
    b[0]=3

The function arguments a and b have no relationship with the global variables a and b. They just happen to have the same names.

b references a list. Lists are mutable, they can be changed. Your function changes the list object so it holds the int object 3 instead of 2.

The "value of" a and b do not change in your program. a always refers to the int object 2, and b always refers to the same list object.

[akbarza]

a and b are just names used to reference objects. The names are of no importance here.

a references an int object. Int objects are immutable, they cannot change. 2 is always 2, and you cannot change 2 to 3. You can reassign a variable to reference a different int object. num first referenced 2, then was assigned to reference 3. The only relationship between a and num is that the int object referred to by a was passed to a function as the argument num. The variables a and num know nothing about each other. Even if you rewrote the function to look like below.

def something(a,b):
    a=3
    b[0]=3

The function arguments a and b have no relationship with the global variables a and b. They just happen to have the same names.

b references a list. Lists are mutable, they can be changed. Your function changes the list object so it holds the int object 3 instead of 2.

The "value of" a and b do not change in your program. a always refers to the int object 2, and b always refers to the same list object.

hi
good explenation and thanks, but can explain more?
thanks again

[deanhystad]

Mutable vs immutable sums it all up. The mutable list object was changed. The immutable int object was not changed. If you have more questions, ask them. I am not going to guess what they might be.