Conditional checks
Validate whether an optional value, collection, or configuration field is present before branching.
Python · Class
bool
bool() converts an object through Python’s standard truth-testing procedure and returns True or False. With no argument it returns False; bool is an int subclass with only the two instances True and False.
- Since
- 3.7
How it works
Calling bool() applies Python's standard truth-testing rules and produces one of the two Boolean objects. With no argument it produces False; the type is also an int subclass, cannot be extended by subclassing, and has only True and False as instances.
Syntax
class bool(object=False, /)Parameters
| Name | Type | Description |
|---|---|---|
object | object=False, positional-only | Object to convert using standard truth testing; omitting it is equivalent to a false value. |
Returns
A bool value: True when the argument is truthy, otherwise False.
Key points
- The call always returns one of the two Boolean values: True or False.
- An argument is evaluated according to Python's standard truth-testing rules before the result is produced.
- False-like input and no argument yield False; every other truthy input yields True.
- Boolean objects inherit from int, which explains their compatibility with integer-oriented operations.
- Applications cannot create a custom subclass of bool.
- There are exactly two bool objects available as Boolean instances.
Example: Convert values to booleans
value = "ready"
if bool(value):
print("truthy")
print(bool([]))
print(bool(0))The non-empty string is true, while an empty list and zero are false.
When to use it
Boolean normalization
Normalize inputs into an explicit Boolean for a flag, predicate result, or serialized state.
Custom truth protocols
Apply Python's truth-testing behavior to custom objects that define __bool__() or __len__().
Interview questions
QWhat does bool() do?
bool() returns either True or False after applying standard truth testing to its argument. Omitting the argument returns False.
Follow-up: What controls truth testing for a user-defined object? Its class can define __bool__(); if that is absent, Python can use __len__() to determine truth.
QHow is bool related to int?
bool is a subclass of int, so Boolean values can behave like 0 and 1 in numeric contexts. Despite that relationship, the documented Boolean instances are only False and True.
Follow-up: How should code make numeric intent explicit? Convert with int() rather than relying on Boolean-to-integer behavior.
QCan bool be subclassed, and how many instances does it have?
The built-in bool type cannot be subclassed further. It has exactly two constant instances: True and False.
Follow-up: What values are false in Boolean contexts? False, None, numeric zero, and empty strings or containers are interpreted as false.
Related
TrueFalseintandornot__bool____len__