__iand__() magic method implements the in-place bitwise AND
x &= y that calculates the result of the bitwise AND operation
x & y, and assigns it to the first operands’ variable
x. This type of in-place operation is also called augmented arithmetic assignment. The method simply returns the new value to be assigned to the first operand.
- When you call
x &= y, Python first attempts to call
- If this is not implemented, it tries the normal bitwise AND operation
- If this is not implemented either, it tries reverse exponentiation operation
y.__rand__(x)with swapped operands.
The result is then assigned to the first operand
x. If none of those operations is implemented, Python raises a
We call this a “Dunder Method” for “Double Underscore Method” (also called “magic method”). To get a list of all dunder methods with explanation, check out our dunder cheat sheet article on this blog.
Basic Example Overriding __iand__
In the following code example, you create a class
Data and define the magic method
- The “self” argument is the default argument of each method and it refers to the object on which it is called—in our case, the first operand of the in-place operation.
- The “other” argument of the in-place method refers to the second operand, i.e.,
yin the in-place operation
x &= y.
The return value of the operation returns a dummy string
'finxter 42' to be assigned to the first operand. In practice, this would be the result of the in-place bitwise AND operation.
class Data: def __iand__(self, other): return 'finxter 42' x = Data() y = Data() x &= y print(x) # finxter 42
In-Place AND &= without __iand__()
To support the in-place bitwise AND operation on a custom class, you don’t have to overwrite the
__iand__() method. Because if the method is not defined, Python will fall back to the normal
__and__() method and assign its result to the first operand.
Here’s an example:
class Data: def __and__(self, other): return 'finxter 42' x = Data() y = Data() x &= y print(x) # finxter 42
Even though the
__iand__() method is not defined, the in-place bitwise AND operation
x &= y still works due to the
__and__() “fallback” magic method!
In-Place AND &= without __iand__() and __and__()
To support in-place bitwise AND
x &= y on a custom class, you don’t even have to overwrite any of the
x.__and__(y) methods. If both are not defined, Python falls back to the reverse
y.__rand__(x) method and assigns its result to the first operand.
Here’s an example where you create a custom class for the first operand that doesn’t support the bitwise AND operation. Then you define a custom class for the second operand that defines the
__rand__() method. For the in-place operation, Python falls back to the
__rand__() method defined on the second operand and assigns it to the first operand
class Data_1: pass class Data_2: def __rand__(self, other): return 'finxter 42' x = Data_1() y = Data_2() x &= y print(x) # finxter 42
TypeError: unsupported operand type(s) for &=
If you try to perform in-place bitwise AND
x &= y but neither
y.__rand(x) is defined, Python raises a “
TypeError: unsupported operand type(s) for &=". To fix this error, simply define any of those methods before performing the in-place operation.
class Data: pass # ... you should define __iand__ here to prevent error! ... # x = Data() y = Data() x &= y
Traceback (most recent call last): File "C:\Users\xcent\Desktop\code.py", line 8, in <module> x &= y TypeError: unsupported operand type(s) for &=: 'Data' and 'Data'
Background Bitwise AND
Python’s bitwise AND operator
x & y performs logical AND on each bit position on the binary representations of integers
y. Thus, each output bit is 1 if both input bits at the same position are 1, otherwise, it’s 0. For example, the integer expression 4 & 3 is translated to binaries 0100 & 0011 which results in 0000 because all four input bit positions are different.
Related Video Compound Operators
Where to Go From Here?
Enough theory. Let’s get some practice!
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