python pointer memory reallocation

Question

I am confused by python's handling of pointers. For example:

a=[3,4,5]
b=a
a[0]=10
print a

returns [10, 4, 5]

The above would indicate that b and a are pointers to the same location. The issue is what if we say:

a[0]='some other type'

b now equals ['some other type', 4, 5]

This is not the behavior I would expect as python would have to reallocate the memory at the pointer location due to the increase in the size of the type. What exactly is going on here?

• Iron Fist: I don't understand...you made b=a, so both a and b are referring to same memory location?...whatever change you do using a will be the same in b ..
• Aslam Khan: please look this link google.co.in/…

Answer 1

1. Variables in Python are just reference to memory location of the object being assigned.
2. Changes made to mutable object does not create a new object

When you do b = a you are actually asking b to refer to location of a and not to the actual object [3, 4, 5]

To explain further:

In [1]: a = [1, 2] 
In [2]: b = a # b & a point to same list object [1, 2] 
In [3]: b[0] = 9 # As list is mutable the original list is altered & since a is referring to the same list location, now a also changes 
In [4]: print a
[9, 2] 
In [5]: print b
[9, 2]

Answer 2

Python does not have pointers. Your code creates another variable b, which becomes a reference to a, in other words, it simply becomes another way of referring to a. Anything you do to a is also done to be, since they are both names for the same thing. For more information, see python variables are pointers?.