I don't understand the difference in the following example. One time an instance of a class can CHANGE the class variable of another instance and the other time it can't?
Example 1:
class MyClass(object):
mylist = []
def add(self):
self.mylist.append(1)
x = MyClass()
y = MyClass()
x.add()
print "x's mylist: ",x.mylist
print "y's mylist: ",y.mylist
Output:
x's mylist: [1]
y's mylist: [1]
So here an instance x of class A was able to access and modify the class attribute mylist which is also an attribute of the instance y of A.
Example 2:
class MyBank(object):
crisis = False
def bankrupt(self) :
self.crisis = True
bankX = MyBank()
bankY = MyBank()
bankX.bankrupt()
print "bankX's crisis: ",bankX.crisis
print "bankY's crisis: ",bankY.crisis
bankX's crisis: True
bankY's crisis: False
Why does this not work in this example?
In first case there is no assignment in add method:
def add(self):
self.mylist.append(1) # NOT self.mylist = something
In second case there is an assignment:
def bankrupt(self) :
self.crisis = True # self.crisis = something
When an attribute is set on instance, it is always set on particular instance only (it's put to instance's __dict__ atribute). Class __dict__ is unaffected.
In first case there is no assignment, so standard look-up rules apply. Since there is no "mylist" in __dict__ attribute of instance, it falls back to class __dict__.
Operation performed in add mutates value stored in MyClass.__dict__. That's why change is observable in all instances.
Consider following snippet (it may explain your issue better):
class MyClass:
x = []
x1 = MyClass()
x2 = MyClass()
x3 = MyClass()
x1.x.append(1)
print x1.x # [1]
print x2.x # [1]
print x3.x # [1]
assert x1.x is x2.x is x3.x
x3.x = "new" # now x3.x no longer refers to class attribute
print x1.x # [1]
print x2.x # [1]
print x3.x # "new"
assert x1.x is x3.x # no longer True!