Python |= In-Place OR Operator

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:

• Python Sets: set union operator
• Python Dictionaries: dictionary update operator
• Python Booleans: logical OR operator

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:

• Python Sets: set union operator
• Python Dictionaries: dictionary update operator
• Python Booleans: logical OR operator

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