Python or Operator

Summary: in this tutorial, you’ll learn about the Python or operator and how to use it effectively.

Introduction to the Python or operator

The or operator is a logical operator. Typically, you use the or operator to combine two Boolean expressions and return a Boolean value.

The or operator returns True if one of the two operands is True. And it returns False only if both operands are False.

This truth table displays the result of the or operator:

The following example shows how to use the or operator:

is_admin = False
is_editor = True
can_edit = is_admin or is_editor

print(can_edit)Code language: PHP (php)

Output:

TrueCode language: PHP (php)

The Python or operator is short-circuiting

When evaluating an expression that involves the or operator, Python can sometimes determine the result without evaluating all the operands. This is called short-circuit evaluation or lazy evaluation.

For example:

x or y

If x is truthy, then the or operator returns x. Otherwise, it returns y.

In other words, if x is truthy, then the or operator doesn’t need to evaluate y. It just returns x immediately. This is why the evaluation is called lazy or short-circuiting evaluation.

The or operator only evaluates y and returns the result of the evaluation if x is falsy.

In Python, every object associates with a Boolean value. And the x and y can be any object.

This opens some useful applications of the or operator.

Setting a default value for a variable

The or operator allows you to set a default value for a variable, for example:

var_name = value or defaultCode language: JavaScript (javascript)

In this example, if value is falsy, the or operator return the default.

The following example prompts you for input. If you don’t enter anything, the lang will default to 'Python':

lang = input('Enter your language:') or 'Python'
print(lang)Code language: PHP (php)

The following example defines a function get_data() that returns a list of numbers. It uses the built-in min() function to find the lowest element in the list:

def get_data(args=None):
    if args:
        return [1, 2, 3]
    return []


lowest = min(get_data(args=true))
print(lowest)Code language: PHP (php)

Output:

1

It returned 1 as expected. However, the get_data() may return an empty list like this:

lowest = min(get_data())
print(lowest)Code language: PHP (php)

It returned a ValueError.

To fix this, you can use the or operator when calling the min() function:

def get_data(args=None):
    if args:
        return [1, 2, 3]
    return []


lowest = min(get_data() or [0])
print(lowest)Code language: PHP (php)

Output:

0

In this example, if the get_data() function returns an empty list, the or operator will treat its result as a falsy value.

Since the first operand is falsy, the or operator needs to evaluate the second operand [0]. In this case, you can specify the default minimum value in the second operand.