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pythonarraysslicemutability

Difference between full variable shallow copying and slice partial copying

From what I understand, Python is a pass by object reference language, which means that if the original value is mutable, every shallow copy will be affected (and vice versa). So, something like:

x = [1,2,3]
y = x
x.append(4)
print(y[-1]) -> 4

Is an expected consequence of the mutability of arrays. But when I make a shallow copy using the slicing operator:

x = [1,2,3]
y = x[:]
x.append(4)
print(y[-1]) -> 3

Why is this behavior happening?

Solution

  • id built-in function will help. As will the is operator.

    x = [1,2,3]
y = x
print(y == x) # True
print(y is x) # True
print(id(y) == id(x)) # True

    So all are true. x and y have the same values, they occupy the same space in memory.

    x = [1,2,3]
y = x[:]
print(y == x) # True
print(y is x) # False
print(id(y) == id(x)) # False

    Only == operator is true. x and y share the same values. However, they exist in two different memory locations.