Python provides the operator `x *= y`

to multiply two objects in-place by calculating the product `x * y`

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

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

in your class definition.

>>> x = 2 >>> x *= 3 >>> x 6

The expression `x *= y`

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

:

>>> x = 2 >>> x = x * 3 >>> x 6

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

## Integer Example

The `*=`

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

>>> x = 2 >>> x *= 21 >>> 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 *= 21.0 >>> x 42.0

## String Example

Can we multiply a string with an integer in-place? Of course! The result is a new string object created by concatenating the first string multiple times as specified by the second integer operand. This is called string concatenation:

>>> x = 'learn! ' >>> x *= 3 >>> x 'learn! learn! learn! '

## List Example

If the first operand is a list, the result of the in-place multiplication operation overwrites an existing list:

>>> my_list = ['Alice', 'Bob'] >>> my_list *= 3 >>> my_list ['Alice', 'Bob', 'Alice', 'Bob', 'Alice', 'Bob']

The in-place multiplication operator on lists 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 multiplication on any other variable pointing to that same object in memory.

>>> my_list = ['Alice', 'Bob'] >>> x = my_list >>> x *= 3 >>> x ['Alice', 'Bob', 'Alice', 'Bob', 'Alice', 'Bob'] >>> my_list ['Alice', 'Bob', 'Alice', 'Bob', 'Alice', 'Bob']

## Incompatible Data Type

What if two operands have an incompatible data type—unlike floats and integers? For example, if you try to multiply a list with a list in place?

>>> x = [1, 2] >>> x *= [1, 2] Traceback (most recent call last): File "<pyshell#12>", line 1, in <module> x *= [1, 2] TypeError: can't multiply sequence by non-int of type 'list'

The result of incompatible addition is a `TypeError`

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

Can you use the operator on custom objects? Yes!

## Python In-Place Multiplication Magic Method

To use the in-place multiplication operator `*=`

on custom objects, define the `__imul__()`

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

and `other`

, updates the first argument `self`

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

In the following code, you multiply two `Data`

objects by combining their contents:

class Data: def __init__(self, data): self.data = data def __imul__(self, other): self.data *= other.data return self x = Data(21) 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 multiplication 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` |

While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.

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