Python __iand__() Magic Method

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Syntax

`object.__iand__(self, other)`

The Python `__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 `x.__iand__(y)`.
• If this is not implemented, it tries the normal bitwise AND operation `x.__and__(y)`.
• 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 `TypeError`.

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 `__iand__(self, other)`.

• 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., `y` in 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.__iand__(y)` or `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 `x`:

```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 `x.__iand__(y)`, nor `x.__and__(y)`, nor `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```

Output:

```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 `x` and `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.

References:

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