What Is The Output Of The Bool() Function If It Is Passed An Empty List?

bool([]) will output False. This behavior is a core concept in Python known as "truthiness," where various values are implicitly interpreted as either True or False in a boolean context.

Python's truth value testing

In Python, the built-in bool() function returns True for "truthy" values and False for "falsy" values. An empty list ([]) is considered a "falsy" value, along with other empty collections and zero-like values.

The conversion from a list to a boolean happens implicitly in several situations, most notably in if and while statements.

Example:

# An empty list is falsy
my_list = []
if my_list:
    print("This list is not empty.")
else:
    print("This list is empty.")
# Output:
# This list is empty.
# A non-empty list is truthy
my_list = [1, 2, 3]
if my_list:
    print("This list is not empty.")
else:
    print("This list is empty.")
# Output:
# This list is not empty.

Use code with caution.

The role of __len__

For objects that have a length, such as lists, tuples, and strings, Python's truth testing procedure relies on the object's __len__() method.

• If an object's __len__() method returns 0, the object is considered "falsy."
• If __len__() returns any non-zero value, the object is "truthy".

Since an empty list [] has a length of 0, bool([]) returns False.

# The length of an empty list is 0
my_list = []
print(len(my_list))
# Output: 0
# Which evaluates to False
print(bool(len(my_list)))
# Output: False

Use code with caution.

Other falsy values in Python

Besides the empty list, other common falsy values in Python include:

• None
• False (of course)
• The integer 0, floating-point 0.0, and complex number 0j
• Empty sequences like '' (empty string) and () (empty tuple)
• Empty collections like {} (empty dictionary) and set() (empty set)

Importance of understanding truthiness

Understanding Python's truthiness rules is crucial for writing concise and readable code. The "Pythonic" way to check if a list is empty is to use its intrinsic boolean value, as if my_list: is more efficient and elegant than if len(my_list) > 0:.

Correct and Pythonic:

if not my_list:
    print("The list is empty.")

Use code with caution.

Alternative (less Pythonic):

if len(my_list) == 0:
    print("The list is empty.")

Use code with caution.

A special case: Nested empty lists

It is important to remember that the truthiness of an object is based on its own emptiness, not the emptiness of its contents. For example, a list containing only empty lists is still considered "truthy" because the outer list itself is not empty.

nested_list = [[], [], []]
# The list itself is not empty, so its boolean value is True
print(bool(nested_list))
# Output: True
if nested_list:
    print("The nested list is not empty.")
# Output: The nested list is not empty.

Use code with caution.