Python object list weird behaviour

Question

I just started learning Python and come from a Java/C++ background and find the following behavior a little confusing. After calling s.add_trick("Woof") and d.add_trick("Woof"), s.tricks and d.tricks both contain ["Woof", "Bark"]. However, calling party() doesn't have the same behavior. Can someone explain?

class PartyAnimal:
    x = 0
    name = ''
    tricks = []

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

    def party(self):
        self.x += 1

    def add_trick(self, trick):
        self.tricks.append(trick)

s = PartyAnimal("Sally")
d = PartyAnimal("Danny")

s.party()
d.party()
s.add_trick("Woof")
d.add_trick("Bark")

print 's', s.name, s.x, s.tricks
print 'd', d.name, d.x, d.tricks

Output is this:

s Sally 1 ['Woof', 'Bark']
d Danny 1 ['Woof', 'Bark']

`tricks` is a class instance so there is only one copy shared by every instance of `PartyAnimal()`. — dawg, Mar 11 '16 at 18:48
In `C++` your variables `x`, `name` and `tricks` would be `static` i.e. there are shared by all instances of your class. To make them specific to an instance of your class, assign them to `self` only. — Cory Kramer, Mar 11 '16 at 18:50
This is covered in the dupe target, but an important difference is that `self.tricks.append(trick)` modifies the existing `tricks` class attribute, but `self.x += 1` does _not_ modify the `x` class attribute because you're performing an assignment. Instead, it creates an `x` instance attribute (which shadows the class attribute of the same name) to hold the new integer object. — PM 2Ring, Mar 11 '16 at 19:22
(cont) If you did `self.tricks = self.tricks + [trick]` that would also create a `tricks` instance attribute, although using `+=` wouldn't, it'd just mutate the class attribute, like the `.append` method does. `+=` on an integer _can't_ mutate the integer because Python integers are immutable. — PM 2Ring, Mar 11 '16 at 19:22

score 5 · Answer 1 · answered Mar 11 '16 at 18:48

5

You're creating one tricks list that is shared by all instances of class. Create a new list in the __init__ instead:

def __init__(self, name):
    self.name = name
    self.tricks = []

answered Mar 11 '16 at 18:48

Antti Haapala -- Слава Україні

129,958
22
279
321

Though not related, is it a good practice to put `tricks = []` at class level, just as people do it in c# or java? – shahkalpesh Mar 11 '16 at 18:50
1

@shahkalpesh No because that is semantically different than having each instance have its own list. Putting the list at the class level would be like making it `static` in C style languages. – Cory Kramer Mar 11 '16 at 18:51
How come changing s.name or s.x doesn't have the same behavior as appending to a list? – Danny Liu Mar 11 '16 at 18:51
1

it is totally OK **if** it is supposed to be a class variable, akin to `static` member variable in Java/C# – Antti Haapala -- Слава Україні Mar 11 '16 at 18:51
1

@DannyLiu because you're not mutating that integer or string, you're setting the `x` or `name` attribute to point to another object instead. – Antti Haapala -- Слава Україні Mar 11 '16 at 18:53
How does `self.tricks.append` work when it isnt initialized, assuming `tricks = []` is more like static? – shahkalpesh Mar 11 '16 at 18:54
that line `self.tricks = []` will create a new list `[]` and set attribute `tricks` of `self` point to that. – Antti Haapala -- Слава Україні Mar 11 '16 at 18:57
@shahkalpesh: `self.tricks = []` is pretty much a shortcut for `self.tricks = list()`. It *is* initialized. – zondo Mar 11 '16 at 18:58
Agree. Where is `self.tricks = []` in the original question? – shahkalpesh Mar 11 '16 at 19:00
@shahkalpesh nowhere. It is in my answer. – Antti Haapala -- Слава Україні Mar 11 '16 at 19:03
@AnttiHaapala: Although I could run the code of *original question*, I cant understand how `self.tricks.append` work assuming `tricks = []` is a class level variable? :) – shahkalpesh Mar 11 '16 at 19:05
1

@shahkalpesh never too much reputation to start [reading Python 3 tutorials](https://docs.python.org/3/tutorial/classes.html#class-and-instance-variables) – Antti Haapala -- Слава Україні Mar 11 '16 at 19:07
same applies to @DannyLiu, your example is very well explained there ^ – Antti Haapala -- Слава Україні Mar 11 '16 at 19:09
2

@AnttiHaapala got it. Coming from c# background, I thought `self` in python is `this` in c#/java. Hence, you can't do `this` to refer to a class level variable (`static` in c#/java). Thank you. I learned something new today. – shahkalpesh Mar 11 '16 at 19:10

Python object list weird behaviour

1 Answers1