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

• Difficulty Level : Easy
• Last Updated : 15 May, 2020

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.

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