bool()

The bool type constructor — the explicit form of the truthiness test that `if` uses implicitly.

Built-in functionPython 2.3+Live demo
Common call
if not bool(items):
Returns
True or False — always
Replaces
writing `if len(x) > 0 else False`-style verbose checks
Watch out
bool("False") is True — the STRING is non-empty; only the empty string is falsy
bool(xx — Any value. No argument returns False. Objects are tested via __bool__() (if defined), then __len__() (0 is falsy), then default truthy.type: Any · default: False=False)
→ bool

Demo

Live evaluation
Try:
Inputs
xstrany value
Output
bool('0')
True

The demo passes text through the input, so most cases test string truthiness — the string "0" and the string "False" are BOTH truthy because they are non-empty. Only the empty string is falsy. In real code bool() also handles numbers, lists, dicts, sets, and custom objects via their __bool__ or __len__ methods.

Parameters

NameTypeRequiredDescription
xAnyno (False)Any value. No argument returns False. Objects are tested via __bool__() (if defined), then __len__() (0 is falsy), then default truthy.

Return value

bool — True or False. Falsy values: 0, 0.0, "", [], (), {}, set(), None, False, and objects whose __bool__ or __len__ returns falsy. Everything else is truthy.

Common patterns

Coerce to strict True/False
Ensures downstream code sees a real bool, not a truthy string or list.
active = bool(user_input)   # store True or False, not the raw value
Filter truthy items
filter(None, iterable) is idiomatic for "keep truthy items".
kept = list(filter(None, items))
Custom __bool__ on a class
Objects can define their own truthiness — falls back to __len__ if absent.
class Cart:
    def __bool__(self):
        return self.total > 0

Examples

1. Zero is False
bool(0)
Returns
False
2. Positive is True
bool(1)
Returns
True
3. Empty string False
bool("")
Returns
False
4. "False" string True
bool("False")
Returns
True # non-empty string
5. Empty list False
bool([])
Returns
False
6. List with 0 True
bool([0])
Returns
True # length > 0
7. None is False
bool(None)
Returns
False
8. No argument
bool()
Returns
False

Pitfalls

1. bool("False") is TRUE
The rule is emptiness, not semantics. The string "False" is a non-empty string — Python does not read English. Parsing user input to a real bool needs an explicit mapping.
Assumed parse
is_admin = bool(request.form["is_admin"])   # user sent "False"
True # non-empty string
Explicit map
is_admin = request.form["is_admin"].lower() == "true"
False when the string says "false"
2. bool([0]) is TRUE
Truthiness of a container tests LENGTH, not contents. A list with a single zero is still non-empty, therefore truthy.
Length-based
if bool(items):
    ...  # runs for [0], [False], [None]
runs even when contents are all falsy
Test contents
if any(items):
    ...  # only if at least one is truthy
runs only when a truthy item exists
3. bool is a subclass of int — True == 1, False == 0
True and False are literal integers under the hood. Arithmetic with bools works — sometimes usefully (sum a list of bools for a count), sometimes surprisingly.
Weird types
True + True + False
2 # integer arithmetic
Use it deliberately
count = sum(x > 0 for x in items)
idiomatic count of matches
4. `is True` and `is False` almost never match user expectations
A truthy value is not the same as the True singleton. `if x is True:` fails for 1, "yes", or any custom truthy object.
Identity check
if authenticated is True:
    ...  # misses truthy 1
branch skipped for 1
Truthiness
if authenticated:
    ...  # standard truthiness
covers all truthy values

When to use

Use it
  • Storing a strict True/False in place of an arbitrary truthy/falsy value
  • Filtering truthy items — filter(None, iterable) or bool() in a comprehension
  • Testing a custom object's __bool__ method explicitly
  • Counting matches — `sum(cond(x) for x in items)` relies on True == 1
Reach for something else
  • Parsing "true" / "false" strings from user input → explicit comparison
  • Testing whether a container has truthy CONTENTS → any() instead
  • `if x is True` — use plain `if x`
  • Comparing custom-object truthiness across types → be explicit

Notes

Complexity
O(1) for most types; O(n) if __len__ walks a collection
Return
bool — always the True or False singleton
CPython impl
Objects/boolobject.c :: bool_new — dispatches to type's __bool__ then __len__
Memory
No allocation — True and False are singletons
Thread-safe
Yes — a pure computation

FAQ

Explicit mapping — bool() would treat both as truthy since they are non-empty strings.

def parse_bool(s):
    return s.strip().lower() in ("true", "yes", "1", "on")

History

2.3
bool type introduced — before this, True and False were plain integers (1 and 0) named in the code.