Python In-Place Assignment 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.

You can watch me go over all of these operators in the following video:

We’ll rush over all in-place operators one-by-one next!

Python In-Place Addition

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

Related Tutorial: Python In-Place Addition

Python In-Place Subtraction

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

Related Tutorial: Python In-Place Subtraction

Python In-Place Multiplication

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

Related Tutorial: Python In-Place Multiplication

Python In-Place Division

Python’s in-place division operator x /= y divides two objects in-place by calculating x / y and assigning the result to the first operands variable name x. Set up in-place division for your own class by overriding the magic “dunder” method __truediv__(self, other) in your class definition.

>>> x = 4
>>> x /= 2
>>> x
2

The expression x /= y is syntactical sugar for the longer-form x = x / y:

>>> x = 4
>>> x = x / 2
>>> x
2

Related Tutorial: Python In-Place Division

Python In-Place Modulo

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

Related Tutorial: Python In-Place Modulo

Python In-Place Integer Division

Python’s in-place integer division operator x //= y divides two objects in-place by calculating x // y and assigning the result to the first operands variable name x. Set up in-place integer (or floor) division for your own class by overriding the magic “dunder” method __floordiv__(self, other) in your class definition.

>>> x = 5
>>> x //= 2
>>> x
2

The expression x /= y is syntactical sugar for the longer-form x = x / y:

>>> x = 5
>>> x = x // 2
>>> x
2

Related Tutorial: Python In-Place Integer Division

Python In-Place Exponentiation

Python provides the in-place exponentiation operator x **= y that raises x to the power of y using x ** y and assigns the result to the first operands variable name x. You can set up the in-place exponentiation behavior for your own class by overriding the magic “dunder” method __ipow__(self, other) in your class definition.

>>> x = 2
>>> x **= 3
>>> x
8

The expression x **= y is syntactical sugar for the longer-form x = x ** y:

>>> x = 2
>>> x = x ** 3
>>> x
8

Related Tutorial: Python In-Place Exponentiation

Python In-Place Bitwise AND

Python’s in-place bitwise AND operator x &= y calcualtes bitwise-and x & y and assigns the result to the first operand x. To set it up for your own class, override the magic “dunder” method __iand__(self, other) in your class definition.

>>> x = 1
>>> x &= 2
>>> x
0

The expression x &= y is syntactical sugar for the longer-form x = x & y:

>>> x = 1
>>> x = x & 2
>>> x
3

Related Tutorial: Python In-Place Bitwise AND

Python In-Place Bitwise OR

Python’s A |= B applies the | operator in place. Thus, it is semantically identical to the longer-form version A = A | B of first performing the operation A | B and then assigning the result to the variable A.

The following minimal example creates two Boolean variables A and B and performs the in-place B |= A operation to perform a logical OR operation B | A and assigning the result to the first operand B that becomes True:

>>> A = True
>>> B = False
>>> B |= A
>>> B
True

In this example, you’ve seen this in-place operation on Boolean operands. But the | operator is overloaded in Python. The three most frequent use cases for the | and |= operators are the following:

• Python Sets: set union operator
• Python Dictionaries: dictionary update operator
• Python Booleans: logical OR operator

Related Tutorial: Python In-Place Bitwise OR

Python In-Place Bitwise XOR

Python’s in-place bitwise XOR operator x ^= y calcualtes bitwise XOR x ^ y and assigns the result to the first operand x. To set this up for your own class, override the magic “dunder” method __ixor__(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

Related Tutorial: Python In-Place Bitwise XOR

Python In-Place Bitwise Right-Shift

Python’s in-place bitwise right-shift operator x >>= y calculates the right-shift operation x >> y, and assigns the result to the first operands variable name x. You can set up the in-place right-shift behavior in your own class by overriding the magic “dunder” method __irshift__(self, other) in your class definition.

>>> x = 8
>>> x >>= 2
>>> x
2

The expression x >>= y is syntactical sugar for the longer-form x = x >> y:

>>> x = 8
>>> x = x >> 2
>>> x
2

Related Tutorial: Python In-Place Bitwise Right-Shift

Python In-Place Bitwise Left-Shift

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

Related Tutorial: Python In-Place Bitwise Left-Shift

Python In-Place Magic Methods

The following table provides the names of the magic methods you need to define to enable in-place operators on your custom class:

In the following code example, we create a custom class Data and define our “magic” double-underscore methods so that we can perform in-place computations on objects of this class.

class Data:

    def __init__(self, data):
        self.data = data

    def __iadd__(self, other):
        self.data += other.data
        return self

    def __isub__(self, other):
        self.data -= other.data
        return self

    def __imul__(self, other):
        self.data *= other.data
        return self

    def __truediv__(self, other):
        self.data /= other.data
        return self

    def __imod__(self, other):
        self.data %= other.data
        return self

    def __floordiv__(self, other):
        self.data //= other.data
        return self

    def __ipow__(self, other):
        self.data **= other.data
        return self

    def __iand__(self, other):
        self.data &= other.data
        return self

    def __ior__(self, other):
        self.data |= other.data
        return self

    def __ixor__(self, other):
        self.data ^= other.data
        return self

    def __irshift__(self, other):
        self.data >>= other.data
        return self

    def __ilshift__(self, other):
        self.data <<= other.data
        return self
    

Let’s try these out!

# In-Place Addition
x = Data(3)
y = Data(2)
x += y
print(x.data)
# 5

# In-Place Subtraction
x = Data(3)
y = Data(2)
x -= y
print(x.data)
# 1

# In-Place Multiplication
x = Data(3)
y = Data(2)
x *= y
print(x.data)
# 6

# In-Place Division
x = Data(3)
y = Data(2)
x /= y
print(x.data)
# 1.5

# In-Place Modulo
x = Data(3)
y = Data(2)
x %= y
print(x.data)
# 1

# In-Place Integer Division
x = Data(3)
y = Data(2)
x //= y
print(x.data)
# 1

# In-Place Power
x = Data(3)
y = Data(2)
x **= y
print(x.data)
# 9

# In-Place Bitwise AND
x = Data(3)
y = Data(2)
x &= y
print(x.data)
# 2

# In-Place Bitwise OR
x = Data(3)
y = Data(2)
x |= y
print(x.data)
# 3

# In-Place Bitwise XOR
x = Data(3)
y = Data(2)
x ^= y
print(x.data)
# 1

# In-Place Bitwise Right-Shift
x = Data(3)
y = Data(2)
x >>= y
print(x.data)
# 0

# In-Place Bitwise Left-Shift
x = Data(3)
y = Data(2)
x <<= y
print(x.data)
# 12

Chris

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 that has taught exponential skills to millions of coders worldwide. He’s the author of the best-selling programming books Python One-Liners (NoStarch 2020), The Art of Clean Code (NoStarch 2022), and The Book of Dash (NoStarch 2022). Chris also coauthored the Coffee Break Python series of self-published books. He’s a 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.