Python provides the operator `x -= y`

to subtract two objects in-place by calculating the difference `x - y`

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

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

in your class definition.

>>> x = 3 >>> x -= 2 >>> x 1

The expression `x -= y`

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

:

>>> x = 3 >>> x = x - 2 >>> x 1

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

Table of Contents

## Integer Example

The `-=`

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

>>> x = 82 >>> 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 -= 10.0 >>> x -8.0

## Set Example

If the operands are sets, the result of the in-place subtraction operation overwrites an existing set with the set difference operation:

>>> my_set = {'Alice', 'Bob', 'Carl'} >>> my_set -= {1, 'Bob', 'Alice'} >>> my_set {'Carl'}

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

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

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

>>> my_set = {'Alice', 'Bob', 'Carl'} >>> x = my_set >>> x -= {1, 'Bob', 'Alice'} >>> x {'Carl'} >>> my_set {'Carl'}

## Incompatible Data Type

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

>>> x = 4 >>> x -= [1, 2, 3] Traceback (most recent call last): File "<pyshell#9>", 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 Subtraction Magic Method

To use the in-place subtraction operator `-=`

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

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

and `other`

, updates the first argument `self`

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

In the following code, you subtract two `Data`

objects from each other by combining their contents:

class Data: def __init__(self, data): self.data = data def __isub__(self, other): self.data -= other.data return self x = Data(44) 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 subtraction 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.

To help students reach higher levels of Python success, he founded the programming education website Finxter.com. He’s author of the popular programming book Python One-Liners (NoStarch 2020), coauthor of the Coffee Break Python series of self-published books, computer science enthusiast, freelancer, and owner of one of the top 10 largest Python blogs worldwide.

His passions are writing, reading, and coding. But his greatest passion is to serve aspiring coders through Finxter and help them to boost their skills. You can join his free email academy here.