Variable assignment and modification (in python) [duplicate]

Question

When I ran this script (Python v2.6):

a = [1,2] b = a a.append(3) print a >>>> [1,2,3] print b >>>> [1,2,3] 

I expected print b to output [1,2]. Why did b get changed when all I did was change a? Is b permanently tied to a? If so, can I make them independent? How?

Answer 1

Memory management in Python involves a private heap memory location containing all Python objects and data structures.

Python's runtime only deals in references to objects (which all live in the heap): what goes on Python's stack are always references to values that live elsewhere.

>>> a = [1, 2] 

>>> b = a 

>>> a.append(3) 

Here we can clearly see that the variable b is bound to the same object as a.

You can use the is operator to tests if two objects are physically the same, that means if they have the same address in memory. This can also be tested also using the id() function.

>>> a is b >>> True >>> id(a) == id(b) >>> True 

So, in this case, you must explicitly ask for a copy. Once you've done that, there will be no more connection between the two distinct list objects.

>>> b = list(a) >>> a is b >>> False 

Answer 2

Objects in Python are stored by reference—you aren't assigning the value of a to b, but a pointer to the object that a is pointing to.

To emulate assignation by value, you can make a copy like so:

import copy  b = copy.copy(a)  # now the code works as "expected" 

Be aware this has performance disadvantages.

In the case of an array, there's a special method that relies on slices:

b = a[:]  # code also works as expected here 

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Update– In addition to this, with some objects you can use the constructor—this includes lists:

b = list(a)