bool()

The built-in bool() function allows you to determine the truth value of any Python object. It returns True or False, based on whether the object is considered truthy or falsy in Python:

Python 
>>> bool(0)
False

>>> bool(42)
True

bool() Signature

Python Syntax 
bool([x])

Arguments

Argument Description Default Value
x The object to be checked. False

Return Value

  • Returns the truth value of any Python object as a Boolean value (True or False).

bool() Examples

With no argument:

Python 
>>> bool()
False

With falsy objects:

Python 
>>> # Built-in constants
>>> bool(None)
False
>>> bool(False)
False

>>> # Numeric zeros
>>> bool(0)
False
>>> bool(0.0)
False
>>> bool(0j)
False

>>> # Empty sequences
>>> bool("")
False
>>> bool([])
False
>>> bool(())
False
>>> bool(range(0))
False

>>> # Empty dictionary and set
>>> bool({})
False
>>> bool(set())
False

With numbers as arguments:

Python 
>>> bool(42)
True
>>> bool(3.14)
True

With a string as an argument:

Python 
>>> bool("Hello")
True

bool() Common Use Cases

The most common use cases for the bool() function include:

  • Evaluating the truthiness of variables or expressions
  • Normalize objects to Boolean values in conditional expressions
  • Implementing custom Boolean logic in classes

bool() Real-World Example

As an example of using bool(), say that you need to implement a stack data structure that should be cosidered falsy if it’s empty and truthy otherwise. You can do the following:

Python stack.py 
class Stack:
    def __init__(self, items=None):
        self.items = list(items) if items is not None else []

    def push(self, item):
        self.items.append(item)

    def pop(self):
        return self.items.pop()

    def __bool__(self):
        return bool(self.items)

# Usage
stack = Stack()
print(bool(stack))  # Output: False
stack.push(4)
print(bool(stack))  # Output: True

In this example, your Stack class implements the .__bool__() special method to support Boolean operations on its objects. This method guarantees that when a given Stack object is empty, the bool() function returns False and True otherwise.

bool() in Custom Classes

As you’ve seen. you can support the bool() function in custom classes by defining the .__bool__() special method. Here’s a quick example:

Python container.py 
class Container:
    def __init__(self, items=None):
        self.items = items or []

    def __bool__(self):
        return len(self.items) > 0

# Usage
container = Container()
print(bool(container))  # Output: False
container.items.append(1)
print(bool(container))  # Output: True

In this example, the Container class uses the .__bool__() method to determine its truthiness based on whether it contains any items.

Python Booleans: Optimize Your Code With Truth Values

Tutorial

Python Booleans: Use Truth Values in Your Code

In this tutorial, you'll learn about the built-in Python Boolean data type, which is used to represent the truth value of an expression. You'll see how to use Booleans to compare values, check for identity and membership, and control the flow of your programs with conditionals.

intermediate python

For additional information on related topics, take a look at the following resources:

By Leodanis Pozo Ramos • Updated Nov. 21, 2024 • Reviewed by Dan Bader
Python’s Built-in Functions Share Feedback