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) |
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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
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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