what is the difference between a = x and a=x[:] in python [duplicate]

Answer 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

Answer 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

Answer 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]
>>>