Prev Next

Python / Core Python Fundamentals Interview Questions

How does Python determine whether a custom object is truthy or falsy?

Every Python object has a boolean value. In a boolean context (an if condition, a while condition, or passed to bool()), Python calls the object's __bool__ method first. If that is not defined, it falls back to __len__ and returns False if __len__ returns 0. If neither is defined, the object is always truthy.

class Queue:
    def __init__(self):
        self._data = []

    def enqueue(self, item):
        self._data.append(item)

    def __len__(self):
        return len(self._data)

    def __bool__(self):
        return len(self._data) > 0   # explicit

q = Queue()
if not q:
    print('Queue is empty')    # printed — __bool__ returns False

q.enqueue('item')
if q:
    print('Queue has items')   # printed — __bool__ returns True

The built-in falsy values to memorise: None, False, 0, 0.0, 0j (complex zero), '' (empty string), b'' (empty bytes), [], (), {}, set(), and any object whose __bool__ returns False or whose __len__ returns 0.

Practical impact: you can write Pythonic guards like if items:, while queue:, and return value or default instead of verbose length checks. The short-circuit operators and and or return one of their operands, not necessarily a bool: 'alice' or 'default' returns 'alice'; '' or 'default' returns 'default'.

If a class defines only __len__, returning 0, what does bool(instance) return?
What does '' or 'default' evaluate to in Python?

More Related questions...

What is Python and what makes it popular for software development? How do variables work in Python, and what does dynamic typing mean? How do conditional statements work in Python? How does the for loop work in Python, and what is the role of range()? When should you use a while loop instead of a for loop in Python? What is a Python list and what operations are most commonly used? What is a tuple in Python and when should you choose it over a list? How do Python dictionaries work and what are the most important operations? What is a Python set and what makes it useful for membership testing? How do you define and call a function in Python? What is variable scope in Python and how does the LEGB rule work? How does exception handling work in Python using try/except? What are the different ways to format strings in Python, and which is preferred? What is list comprehension and how does it differ from a regular for loop? What are *args and **kwargs in Python function definitions? What is a lambda function in Python and when is it appropriate to use one? What is the mutable default argument trap in Python and how do you fix it? How do you use dictionary comprehension to transform data payloads in Python? How does slicing work in Python for lists, strings, and tuples? What is tuple unpacking and extended unpacking in Python? What do the pass, break, and continue statements do in Python loops? What is None in Python, and when should you use 'is' versus '=='? How do you work with nested data structures such as a list of dictionaries? What is a generator in Python and how does it differ from a list? What is a decorator in Python and how do you write one? How do you define a class in Python, and what is the role of __init__? How does inheritance work in Python and what is method resolution order (MRO)? How do you read from and write to files in Python? How do Python modules and imports work? Which Python built-in functions are most important to know for coding interviews? How does Python determine whether a custom object is truthy or falsy? What is the difference between a shallow copy and a deep copy in Python? Which Python string methods are most useful for cleaning and parsing data payloads? How do enumerate() and zip() make loops more Pythonic? What is the Python exception class hierarchy and how do you create custom exceptions? What is the walrus operator (:=) and when is it useful? How does Python's sort work, and what is the difference between sort() and sorted()? What are Python dataclasses and when should you use them instead of regular classes? What is a context manager in Python and how do you implement one? How do Python type hints work and how do you use them in function signatures? Can you create a tuple comprehension in Python, and what is a generator expression? How does recursion work in Python and what are its limitations? How do you parse and build JSON payloads in Python? Why should you use Python's logging module instead of print() in production code? What is PEP 8 and which conventions does it define for Python code?
Show more question and Answers...

Data Science Essentials Interview Questions

Comments & Discussions

Interviews Questions

About Javapedia.net

Javapedia.net is for Java and J2EE developers, technologist and college students who prepare of interview. Also this site includes many practical examples.

This site is developed using J2EE technologies by Steve Antony, a senior Developer/lead at one of the logistics based company.

Follow Javapedia updates

contact: javatutorials2016[at]gmail[dot]com

Kindly consider donating for maintaining this website. Thanks.

Copyright © 2026, javapedia.net, all rights reserved. · .

DMCA.com Protection Status