class A:
def __init__(self, n=[0]):
self.data = n
a = A()
print a.data[0] #print 0
a.data[0] +=1
b = A()
print a.data[0] #print 1, desired output is 0
In the case above, is there any way to provide a default argument with the mutable object (such as list or class) in __init__() class A, but b is not affected by the operation a?
You could try this:
class A:
def __init__(self, n=None):
if n is None:
n = [0]
self.data = n
Which avoids the biggest problem you're facing here, that is, that's the same list for every single object of your type "A."
One possibility is:
class A:
def __init__(self, n=None):
if n is None:
n = [0]
self.data = n
Also:
class A:
def __init__(self, n=[0]):
print id(n)
self.data = n[:]
print id(self.data)
del n
a = A()
print a.data[0] #prints 0
a.data[0] +=1
print a.data[0] #prints 1
print
b = A()
print b.data[0] #prints desired output 0
The principle is that it creates another list. If a long list is passed as argument, there will be two long list in memory. So the inconvenience is that it creates another list.... That's why I delete n.
Don't think it's better, but it may give you comprehension of what happens
