# Python |= In-Place OR Operator

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Have you stumbled upon the strange-looking Python operator `A |= B` in a code snippet and you don’t know what it means? This article will clarify it once and for all! Let’s start with the short answer:

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:

Let’s dive into each of them one by one.

## |= on Python Sets

Python’s `set.union(set_1, set_2, ...)` creates and returns a new set consisting of the elements that are members of any of the involved sets. A shorthand notation for the set union operator is the `|` operator such as in `set_1 | set_2 | set_3`. The `set_1 |= set_2` operator performs the set union operator in-place—it replaces the set given as a first operand.

For example, the following three expressions are semantically equivalent—they all perform the union of sets `set_1` and `set_2` and assign the result to the `set_1` variable.

```>>> set_1 = set_1 | set_2
>>> set_1 |= set_2
>>> set_1.__ior__(set_2)```

The first operation is an assigned OR operation. The second operation is an in-place OR. The third is an in-place operation using a special “dunder” method.

The following minimal example shows how `set_1` is updated with the union of the two sets, in-place:

```>>> set_1 = {'Alice'}
>>> set_2 = {'Bob', 'Alice', 1, 2, 3}
>>> set_1 |= set_2
>>> set_1
{1, 2, 3, 'Bob', 'Alice'}```

## |= on Dictionaries

Python 3.9 has introduced the merge and update operators on dictionaries.

• `dict_1 | dict_2` creates a new dictionary with all elements in `dict_1` and `dict_2`. The second operand takes precedence over the first, so if a key exists in both dictionaries, Python uses the (key, value) pair from the second dictionary.
• `dict_1 |= dict_2` updates the first dictionary `dict_1` with the same merged dictionary elements.

In the following example, we updated the first dictionary with the (key, value) pairs from the second dictionary:

```d1 = {'Alice': 42, 'Bob': 18}
d2 = {'Alice': 18, 'Carl': 22}
d1 |= d2
print(d1)```

The output is the updated dictionary

`{'Alice': 18, 'Bob': 18, 'Carl': 22}`

## |= on Booleans

The Python `|=` operator when applied to two Boolean values `A` and `B` performs the logical OR operation `A | B` and assigns the result to the first operand `A`. As a result, operand `A` is `False` if both `A` and `B` are `False` and `True` otherwise.

This is shown in the following example where variable B is updated with the result of the operation `B | A` using the in-place Boolean OR operation `B |= A`.

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

## Python In-Place 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.

## Summary

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 | operator is most often used as one of the following:

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