Why different datatypes as class attribute behave differently?

Question

Why is it that when I assign a new value to class attribute which is of type 'int', it does not affect new instances of the class. While when I change value of a type 'list', all of the instances that will be created will follow?

Consider class Human:

class Human:
    health = 100
    posessions = []

    def __init__(self, name):
        self.name = name

And execution:

print("Instatiate object anne as Human: ")
anne = Human("Anne")
# print anne instance's health attribute, I expect:
# Health = 100, Posessions = 0
print("anne.name = {}".format(anne.name))
print("anne.health = {}".format(anne.health)) # OK
print("Number of posessions= ".format(len(anne.posessions)), end="\n\n")

print("Change object anne's attributes: ")
# change and print anne instance's health attribute, I expect:
# Health = 95, Possessions: 3
anne.health = 95
anne.posessions.append("guitar")
anne.posessions.append("notebook")
anne.posessions.append("phone")
print("anne.name = {}".format(anne.name))
print("anne.health = {}".format(anne.health))# OK
print(len(anne.posessions)) # OK
print("Number of posessions: ".format(len(anne.posessions)))
for ps in  anne.posessions:
    print(ps)
print("\n")

print("Create new instance carlou as Human")
carlou = Human("Carlou")
# print carlou instance's health attribute, I expect
# Health: 100, Possession = 0
print("carlou.name = {}".format(carlou.name))
print("carlou.health = {}".format(carlou.health)) # OK
print(len(carlou.posessions)) # ???
print("Number of posessions: ".format(len(carlou.posessions))) # ???
for ps in  carlou.posessions:
    print(ps)

Results:

Instatiate object anne as Human: 
anne.name = Anne
anne.health = 100
Number of posessions= 

Change object anne's attributes: 
anne.name = Anne
anne.health = 95
3
Number of posessions: 
guitar
notebook
phone


Create new instance carlou as Human
carlou.name = Carlou
carlou.health = 100
3
Number of posessions: 
guitar
notebook
phone

Ideally, in results, I expect the length of carlou instance's posession is 0, but it carried anne's length and content.

Could somebody clarify the reason behind this?

Thanks!

Answer 1 · 2017-10-01T06:10:37Z

October 1, 2017 6:10am

I'm unclear on all the aspects of classes, but it is the way the list is initialized. If you move posessions into the init, so it gets initialized when a new instance is created it seems to work. If I was to guess why health and posessions are different. Most variable are just pointers to memory locations. Health assigns a value every time, but posessions is open. It may be passing the memory location that is assigned to that variable.

class Human:
    health = 100
    def __init__(self, name):
        self.name = name
        self.posessions = []

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annecalija

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Why different datatypes as class attribute behave differently?

1 Answer

William Harrison

William Harrison

annecalija

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