# Python __lshift__() Magic Method

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## Syntax

`object.__lshift__(self, other)`

The Python `__lshift__()` method implements the built-in `<<` operation. So, when you cal `x << y`, Python attempts to call `x.__lshift__(y)`. If the method is not implemented, Python first attempts to call `__rlshift__` 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 lshift()

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`.

```print(8 << 1)
# 16

print(8 << 2)
# 32

print(-3 << 1)
# -6```

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

## Example Custom __lshift__()

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

```class Data:

def __lshift__(self, other):
return '... my result of lshift...'

a = Data()
b = Data()

print(a << b)
# ... my result of lshift...
```

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

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

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

```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 `__lshift__()` 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 `__lshift__(self, other)` method in your class definition as shown previously:

```class Data:

def __lshift__(self, other):
return '... my result of lshift...'```

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

## Python __lshift__ vs __rlshift__

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

`print(x << y)`

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

1. The method `x.__lshift__()` is not implemented in the first place, or
2. The method `x.__lshift__()` 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.__rlshift__()` for reverse left-shift on the right operand `y`. Not that this is not the same as right-shift, it just means that the left-shift operation is called on the second operand `y`.

If the reverse left-shift 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.__lshift__(x)` instead of `x.__lshift__(y)`, the result would be wrong because the operation is non-commutative. That’s why `y.__rlshift__(x)` is needed.

So, the difference between `x.__lshift__(y)` and `x.__rlshift__(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 __rlshift__(self, other):
return 'called reverse lshift'

x = Data_1()
y = Data_2()

print(x << y)
# called reverse lshift
```

References:

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