Python a += b is not always a = a + b

 a += b is not always same as a = a + b in Python

In Python A += B does not always behave the same way A = A+B, the same operand can give different results in different situations.

Lets see this by an example

Example :

list1 = [6, 5, 4, 3, 2, 1]

list2 = list1

list1 += [1, 2, 3, 4, 5]

  

print(list1)

print(list2)




Output:

[6, 5, 4, 3, 2, 1, 1, 2, 3, 4, 5]
[6, 5, 4, 3, 2, 1, 1, 2, 3, 4, 5]

Example :

list1 = [6, 5, 4, 3, 2, 1]

list2 = list1

list1 = list1 + [1, 2, 3, 4. 5]

  

print(list1)

print(list2)




Output:

[6, 5, 4, 3, 2, 1, 1, 2, 3, 4, 5]
[6, 5, 4, 3, 2, 1]








expression is list1 += [1, 2, 3, 4, 5] actually modifies the list in-place, means it extends the list such that “list1” and “list2” still have the reference to the same list.


expression is list1 = list1 + [1, 2, 3, 4, 5] creates a new list and changes “list1” reference to that new list and “list2” still refer to old list.







In this article i have explained that a += b is not always a = a + b 

























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