问题
I am trying to learn Python. Can someone please help me understand the difference between following two: a = x vs a=x[:]
回答1:
a = x creates a reference:
a = [2]
x = a
print id(a)
print id(x)
Produces:
39727240
39727240
So if you alter a then x would change too because they are the same objects
Whereas
a = x[:] creates a new object
a = [2]
x = a[:]
print id(a)
print id(x)
Produces:
41331528
39722056
But over here changing a doesn't alter x because they are different objects
回答2:
In [648]: b = a
In [649]: b[0] = 2
In [650]: a
Out[650]: [2] <- a also changed
In [651]: b = a[:] <- Creating new object
In [652]: b[0] = 3
In [653]: b
Out[653]: [3]
In [654]: a
Out[654]: [2] <- a did not change
回答3:
Trying to explain:
>>> x = [1,2,3]
>>> a = x
>>> # a is reference to x - a[0] and x[0] are same variable
>>> a[0] = 4 # same as x[0]...
>>> print x # proof
[4, 2, 3]
>>> a = x[:] # a is copy of x
>>> a[2] = 5 # a[2] is not x[2]
>>> print x
[4, 2, 3]
>>> print a
[4, 2, 5]
>>>
来源：https://stackoverflow.com/questions/20586230/what-is-the-difference-between-a-x-and-a-x-in-python