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
list1 = [ 5 , 4 , 3 , 2 , 1 ] list2 = list1 list1 + = [ 1 , 2 , 3 , 4 ] print (list1) print (list2) |
Output:
[5, 4, 3, 2, 1, 1, 2, 3, 4] [5, 4, 3, 2, 1, 1, 2, 3, 4]
Example 2
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) |
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.
Attention geek! Strengthen your foundations with the Python Programming Foundation Course and learn the basics.
To begin with, your interview preparations Enhance your Data Structures concepts with the Python DS Course.