Python provides the operator x %= y
to calculate the modulo operation x % y
, and assign the result in-place to the first operands variable x
. You can set up the in-place modulo behavior for your own class by overriding the magic “dunder” method __imod__(self, other)
in your class definition.
>>> x = 9 >>> x %= 4 >>> x 1
The expression x %= y
is syntactical sugar for the longer-form x = x % y
:
>>> x = 9 >>> x = x % 4 >>> x 1
Let’s explore some examples on different data types of the operands.
Integer Example
The %=
operator on integer operands stores the remainder of the division of both operands in the left-hand operands’ variable name.
>>> x = 42 >>> x %= 40 >>> x 2
Float Example
If at least one of the operands is a float value, the result is also a float—float is infectious!
>>> x = 42 >>> x %= 40.0 >>> x 2.0
Incompatible Data Type
What if two operands have an incompatible data type—unlike floats and integers? For example, if you try to calculate in-place modulo of two list variables?
>>> [1, 2] % [3, 4] Traceback (most recent call last): File "<pyshell#16>", line 1, in <module> [1, 2] % [3, 4] TypeError: unsupported operand type(s) for %: 'list' and 'list'
The result of incompatible operation is a TypeError
. You can fix it by using only compatible data types for the in-place modulo operation.
Can you use in-place modulo on custom objects? Yes!
Python In-Place Modulo Magic Method
To use the in-place modulo operator %=
on custom objects, you need to define the __imod__()
method (“dunder method”, “magic method”) that takes two arguments self
and other
, updates the first argument self
with the remainder of the division, and returns the updated object.
In the following code, you use the in-place modulo on two Data
objects by defining a custom __imod__()
method:
class Data: def __init__(self, data): self.data = data def __imod__(self, other): self.data %= other.data return self x = Data(40) y = Data(11) x %= y print(x.data) # 7
You can see that the content of the first operand is updated as a result of the in-place modulo operation.
Modulo Video Explanation [Background]
Related Article: Python Modulo Operator
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 |

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