# Python __and__() Magic Method

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## Syntax

`object.__and__(self, other)`

The Python `__and__()` method implements the built-in Bitwise AND `&` operation. So, when you cal `x & y`, Python attempts to call `x.__and__(y)`. If the method is not implemented, Python first attempts to call `__rand__` on the right operand and if this isn’t implemented either, it 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.

## 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.

In this example, you apply the bitwise AND operator to two integers 32 and 16:

```>>> 32 & 16
0```

The expression `32 & 16` operates on the bit representations `"010000"` (decimal 32) and `"001000"` (decimal 16) and performs bitwise AND. As all i-th bit positions are different, the result is 0:

To understand this operation in detail, feel free to read over our tutorial or watch the following video:

## Example Custom __and__()

In the following example, you create a custom class `Data` and overwrite the `__and__()` method so that it returns a dummy string when trying to calculate the bitwise AND operation.

```class Data:

def __and__(self, other):
return '... my result of AND ...'

a = Data()
b = Data()

print(a & b)
# ... my result of AND ...
```

If you hadn’t defined the `__and__()` method, Python would’ve raised a `TypeError`.

## TypeError: unsupported operand type(s) for &

Consider the following code snippet where you try to calculate the operation on custom objects without defining the dunder method `__and__()`:

```class Data:
pass

a = Data()
b = Data()

print(a & b)```

Running this leads to the following error message on my computer:

```Traceback (most recent call last):
File "C:\Users\xcent\Desktop\code.py", line 8, in <module>
print(a & b)
TypeError: unsupported operand type(s) for &: 'Data' and 'Data'```

The reason for this error is that the `__and__()` method has never been defined—and it is not defined for a custom object by default. So, to resolve the `TypeError: unsupported operand type(s) for &`, you need to provide the `__and__(self, other)` method in your class definition as shown previously:

```class Data:

def __and__(self, other):
return '... my result of AND ...'```

Of course, you’d use another return value in practice as explained in the “Background” section.

## Python __and__ vs __rand__

Say, you want to calculate the `&` operation on two custom objects `x` and `y`:

`print(x & y)`

Python first tries to call the left object’s `__and__()` method `x.__and__(y)`. But this may fail for two reasons:

1. The method `x.__and__()` is not implemented in the first place, or
2. The method `x.__and__()` is implemented but returns a `NotImplemented` value indicating that the data types are incompatible.

If this fails, Python tries to fix it by calling the `y.__rand__()` for reverse bitwise AND on the right operand `y`.

If the reverse bitwise AND method is implemented, Python knows that it doesn’t run into a potential problem of a non-commutative operation. If it would just execute `y.__and__(x)` instead of `x.__and__(y)`, the result would be wrong because the operation may be non-commutative when defined as a custom operation. That’s why `y.__rand__(x)` is needed.

So, the difference between `x.__and__(y)` and `x.__rand__(y)` is that the former calculates `x & y` whereas the latter calculates `y & x` — both calling the respective method defined on the object `x`.

You can see this in effect here where we attempt to call the operation on the left operand `x`—but as it’s not implemented, Python simply calls the reverse operation on the right operand `y`.

```class Data_1:
pass

class Data_2:
def __rand__(self, other):
return 'called reverse bitwise AND'

x = Data_1()
y = Data_2()

print(x & y)
# called reverse bitwise AND
```

References:

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