why python associative map member variable is shared between objects

Question

For class A, why is aMap member variable being shared between object and object b?

>>> class A:
...     aMap = {}

>>> a = A()
>>> a.aMap["hello"] = 1

>>> b = A()
>>> b.aMap["world"] = 2

>>> c = []
>>> c.append(a)
>>> c.append(b)

>>> for i in c:
...     for j in i.aMap.items():
...         print j
('world', 2)  
('hello', 1)  
('world', 2)  
('hello', 1)

I believe http://stackoverflow.com/questions/1132941/least-astonishment-in-python-the-mutable-default-argument explains this. — aem, Feb 19 '10 at 21:47
possible duplicate of [How do I avoid having Python class data shared among instances?](http://stackoverflow.com/questions/1680528/how-do-i-avoid-having-python-class-data-shared-among-instances) — Martijn Pieters, Feb 23 '13 at 15:41

score 5 · Accepted Answer · answered Feb 19 '10 at 21:47

5

Because you defined it as a class attribute, not instance attribute.

If you wish to have it as instance attribute and not be shared between instances, you have to define it like this:

class A(object):
    def __init__(self):
        self.aMap = {}

answered Feb 19 '10 at 21:47

gruszczy

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+1: Class attributes are shared among objects. Be careful of vague phrases like "member variable". Try to talk about "instance attributes" and "class attributes", not "member variables", which is vague. – S.Lott Feb 19 '10 at 21:48
True, especially since it's the only way to use properties, when you need them in the future :-) – gruszczy Feb 19 '10 at 21:50

score 3 · Answer 2 · answered Feb 19 '10 at 21:48

Because its a class attribute, not an instance attribute ("member variable").

To make it a instance attribute, assign it on an instance, e.g. in the constructor:

class A:
    def __init__(self):
        self.aMap = {}

But you could also do:

a = A()
a.aMap = {}

score 0 · Answer 3 · answered Feb 19 '10 at 21:51

Because you are effectively defining a class variable instead of an instance variable.

To define an instance variable, you should do it on a method referring the object itself (generally self):

class A(object):
    class_variable = 0

    def __init__(self, a, b):
        self.instance_variable = a
        class_variable = b

>>> a=A(1,2)
>>> print a.instance_variable
1
>>> print a.class_variable
2
>>> b=A(2,1)
>>> print b.instance_variable
2
>>> print b.class_variable
1
>>> print a.instance_variable
1
>>> print a.class_variable
1

why python associative map member variable is shared between objects

3 Answers3