CODEWITHSUNDEEP
pythonarrayspython-2.7slice
Burak.
Burak.

Reputation: 598

Python slice shows same id location

I'm total beginner to Python, so please could you explain me why the following situation happens. Consider the following code:

>>> A = [1, 2, 3, 4]
>>> B = A[0:2]
>>> print id(A) == id(B)
False
>>> print id(A[0]) == id(B[0])
True                              #Why?
>>> A[0] = 9
>>> A
[9, 2, 3, 4]
>>> B 
[1, 2]
>>> print id(A[0]) == id(B[0])
False                             #Contradiction?

As you can see from the code above, I slice the list A and copy it to B, but, why print id(A[0]) == id(B[0]) evalutes Trueon the first one but the opposite when I change either of A or B's value?

Upvotes: 3

Views: 145

Answers (3)

Kasravnd
Kasravnd

Reputation: 107337

Basically, python doesn't create new objects from every item within a list, whenever you copy/slice a it.

But, Although this won't cause any trouble with immutable objects, you should be careful with mutable objects:

In [22]: A = [[1, 2], 2, 3, 4]

In [23]: B = A[0:2]

In [24]: id(A[0]) == id(B[0])
Out[24]: True

In [27]: A[0][1] = 99

In [28]: B
Out[28]: [[1, 99], 2]

In this case you can use copy.deepcopy to create a new object of the slice:

In [32]: import copy

In [33]: B = copy.deepcopy(A[0:2])

In [34]: A[0][1] = 5

In [35]: B
Out[35]: [[1, 99], 2]

In [36]: id(A[0]) == id(B[0])
Out[36]: False
           ^
        New Object

Upvotes: 1

B. Eckles
B. Eckles

Reputation: 1644

When you do B = A[0:2], that ends up essentially doing this, as part of it: B[0] = A[0]. So the object (the integer 1) in A[0] is the same object which is in B[0].

When you set A[0] = 9, then those objects are no long the same.

Also, as @ŁukaszRogalski pointed out CPython caches small integers. So we've got A[0] == 1 == B[0], and id(1) == id(1).

When A[0] == 9, then 9 != 1 == B[0], and id(9) != id(1).

Upvotes: 2

Shasha99
Shasha99

Reputation: 1916

Try this:

id(1) == id(1) #True

The reason is that these number constants will be reused throughout the program. So its like some memory is given to store 1 and then wherever 1 is mentioned in the program, the same memory will be used so only a reference to that memory will be created. Object remains the same.

Upvotes: 2

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