Why the print out looks like this?

Question

class Person:
    pets=[]
    def add_pet(self, pet):
        self.pets.append(pet)

jane = Person()
bob = Person()

jane.add_pet("cat")
print(jane.pets)
print(bob.pets)

Print out:

['cat']
['cat']

why bob's pets is also cats? I didn't append pets to bob.

because `pets` is not an instance member, but a class member. — Jean-François Fabre, Mar 06 '18 at 07:57
because Jane and Bob are married and they both own the `pet`. — Joseph D., Mar 06 '18 at 07:57

score 0 · Answer 1 · answered Mar 06 '18 at 07:58

0

Because you share the variable pets with every instance of Person.

class Person:
    def __init__(self):
        self.pets = []

    def add_pet(self, pet):
        self.pets.append(pet)

This should do the trick.

answered Mar 06 '18 at 07:58

pask

899
9
19

score 0 · Answer 2 · answered Mar 06 '18 at 07:58

The list pets is shared between all objects in the class because of where you declare it. You need a constructor, called __init__, which is called automagically when the object is created:

class Person:
    def __init__(self):
        self.pets = []

    def add_pet(self, pet):
        self.pets.append(pet)


jane = Person()
bob = Person()

jane.add_pet("cat")
print(jane.pets)
print(bob.pets)

Gives:

['cat']
[]

score 0 · Answer 3 · answered Mar 06 '18 at 07:58

0

If you're expecting the pets to be an instance member:

class Person:
    def __init__(self):
        self.pets = []
    ...

Output:

['cat']
[]

answered Mar 06 '18 at 07:58

Joseph D.

11,804
3
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Why the print out looks like this?

3 Answers3