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

In python a += b doesn’t always behave the same way as a = a + b, same operands may give the different results under different conditions.

Consider these examples for list manipulation:
Example 1

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list1 = [5, 4, 3, 2, 1]
list2 = list1
list1 += [1, 2, 3, 4]
  
print(list1)
print(list2)

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Output:


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





Example 2











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


list2 = list1 


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


  


# Contents of list1 are same as above  


# program, but contents of list2 are 


# different. 


print(list1) 


print(list2) 



















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Output:


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





 


    

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

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




    

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