Python provides the operator `x += y`

to add two objects in-place by calculating the sum `x + y`

and assigning the result to the first operands variable name `x`

. You can set up the in-place addition behavior for your own class by overriding the magic “dunder” method `__iadd__(self, other)`

in your class definition.

>>> x = 1 >>> x += 2 >>> x 3

The expression `x += y`

is syntactical sugar for the longer-form `x = x + y`

:

>>> x = 1 >>> x = x + 2 >>> x 3

Let’s explore some examples on different data types of the operands.

## Integer Example

The `+=`

operator on integer operands stores the mathematical sum of both operands in the left-hand operands’ variable name.

>>> x = 2 >>> x += 40 >>> x 42

## Float Example

If at least one of the operands is a float value, the result is also a float—float is infectious!

>>> x = 2 >>> x += 40.0 >>> x 42.0

## String Example

Can we add strings in-place? Of course! The result is a new string object created by concatenating the second string to the first. This is called string concatenation:

>>> x = 'learn' >>> x += ' python' >>> x 'learn python'

## List Example

If the operands are lists, the result of the in-place addition operation overwrites an existing list:

>>> my_list = ['Alice', 'Bob'] >>> my_list += [1, 2, 3] >>> my_list ['Alice', 'Bob', 1, 2, 3]

The in-place add operator on strings doesn’t create a new list object but works on an existing list. Changing the list in-place for one variable `x`

has side-effects. For instance, another variable `my_list`

may point to the same object in memory that is updated through the use of in-place add on any other variable pointing to that same object in memory.

>>> my_list = ['Alice', 'Bob'] >>> x = my_list >>> x += [1, 2, 3] >>> x ['Alice', 'Bob', 1, 2, 3] >>> my_list ['Alice', 'Bob', 1, 2, 3]

## Incompatible Data Type

What if two operands have an incompatible data type—unlike floats and integers? For example, if you try to add a list to an integer variable?

>>> x = 4 >>> x += [1, 2, 3] Traceback (most recent call last): File "<pyshell#75>", line 1, in <module> x += [1, 2, 3] TypeError: unsupported operand type(s) for +=: 'int' and 'list'

The result of incompatible addition is a `TypeError`

. You can fix it by using only compatible data types for the in-place addition operation.

Can you use the addition* *operator on custom objects? Yes!

## Python In-Place Addition Magic Method

To use the in-place addition operator `+=`

on custom objects, you need to define the `__iadd__()`

method (*“dunder method”, “magic method”*) that takes two arguments `self`

and `other`

, updates the first argument `self`

with the result of the addition, and returns the updated object.

In the following code, you add two `Data`

objects together by combining their contents:

class Data: def __init__(self, data): self.data = data def __iadd__(self, other): self.data += other.data return self x = Data(40) y = Data(2) x += y print(x.data) # 42

You can see that the content of the first operand is updated as a result of the in-place add operation.

## Python In-Place Operators

In-place assignment operators (also called *compound* assignment operators) perform an operation in-place on a variable provided as first operand. They overwrite the value of the first operand variable with the result of the operation when performing the operator without assignment. For example, `x += 3`

is the same as `x = x + 3`

of first calculating the result of `x +3`

and then assigning it to the variable x.

Operator | Name | Short Example | Equivalent Long Example |
---|---|---|---|

`=` | In-place Assignment | `x = 3` | |

`+=` | In-place Addition | `x += 3` | `x = x + 3` |

`-=` | In-place Subtraction | `x -= 3` | `x = x - 3` |

`*=` | In-place Multiplication | `x *= 3` | `x = x * 3` |

`/=` | In-place Division | `x /= 3` | `x = x / 3` |

`%=` | In-place Modulo | `x %= 3` | `x = x % 3` |

`//=` | In-place Integer Division | `x //= 3` | `x = x // 3` |

`**=` | In-place Power | `x **= 3` | `x = x ** 3` |

`&=` | In-place Bitwise And | `x &= 3` | `x = x & 3` |

`|=` | In-place Bitwise Or | `x |= 3` | `x = x | 3` |

`^=` | In-place Bitwise XOR | `x ^= 3` | `x = x ^ 3` |

`>>=` | In-place Bitwise Shift Right | `x >>= 3` | `x = x >> 3` |

<<= | In-place Bitwise Shift Left | `x <<= 5` | `x = x << 5` |