Shallow vs. deep copying in Python

Shallow vs. deep copying in Python

If you’ve worked with Lists in Python before, you’ll quickly realise that they work differently from primitives like integers and strings. Consider the following:

a = "hello"
b = a
a = "world"
print(a) # Outputs world
print(b) # Outputs hello

Notice that changing the value of a does not change the value of b. This is called passing by value. In Python, Lists do not behave this way:

a = [2, 3, 4, 5]
b = a
a.append(6)
print(a) # Outputs [2, 3, 4, 5, 6]
print(b) # Outputs [2, 3, 4, 5, 6]

In the above example, notice that changing the value of List a also changes the value of List b. This is because both a and b are referring to the same List, and this is called passing by reference.

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