Python’s in-place bitwise left-shift operator x <<= y
calculates the left-shift operation x << y
, and assigns the result to the first operands variable name x
. You can set up the in-place left-shift behavior in your own class by overriding the magic “dunder” method __ilshift__(self, other)
in your class definition.
>>> x = 8 >>> x <<= 2 >>> x 32
The expression x <<= y
is syntactical sugar for the longer-form x = x << y
:
>>> x = 8 >>> x = x << 2 >>> x 32
Let’s explore some examples on different data types of the operands.
Recap Bitwise Left-Shift
The Python bitwise left-shift operator x << n
shifts the binary representation of integer x
by n
positions to the left. For a positive integer, it inserts a 0
bit on the right and shifts all remaining bits by one position to the left. For example, if you left-shift the binary representation 0101
by one position, you’d obtain 01010
. Semantically, the bitwise left-shift operator x << n
is the same as multiplying the integer x
with 2**n
.

Here’s a minimal example:
print(8 << 1) # 16 print(8 << 2) # 32 print(-3 << 1) # -6
Feel free to watch my explainer video here:
Related. To learn more about the bitwise left-shift operator, and how it works on positive and negative integers, check out our related tutorial: Python Bitwise Left-Shift Operator
Incompatible Data Type
What if two operands have an incompatible data type—unlike floats and integers? For example, if you try to shift a float variable by a list variable (which doesn’t make sense)?
>>> x = 3.0 >>> y = [1, 2] >>> x << y Traceback (most recent call last): File "<pyshell#11>", line 1, in <module> x << y TypeError: unsupported operand type(s) for <<: 'float' and 'list'
The result of incompatible addition is a TypeError
. You can fix it by using only compatible data types for the in-place bitwise left-shift operation.
Can you use the bitwise left-shift operator on custom objects? Yes!
Python In-Place Bitwise Left-Shift Magic Method
To use the in-place bitwise left-shift operator <<=
on custom objects, you need to define the __ilshift__()
method (“dunder method”, “magic method”) that takes two arguments self
and other
, updates the first argument self
with the result of the operation, and returns the updated object.
In the following code, you combine two Data
objects using the in-place left-shift operation:
class Data: def __init__(self, data): self.data = data def __ilshift__(self, other): self.data <<= other.data return self x = Data(8) y = Data(2) x <<= y print(x.data) # 32
You can see that the content of the first operand is updated as a result of the in-place bitwise left-shift 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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