What values are falsy in Python? | Interview Question

Q: What values are falsy in Python?

**Target Audience:** Beginners preparing for Python interviews — no prior Python knowledge assumed. **Difficulty:** Easy **Python version:** Behaviour checked on CPython 3.11 (python3.11 and the in-page Pyodide 0.25.1 runtime, which is CPython 3.11.3); one version-dependent case was checked on 3.12 and 3.13 as noted. > **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. Every output shown in the **Verified** section was copied from a real run of the examples on this page. ## 1. Why This Even Matters — A Story First A doorman at a club checks one thing: is the guest list empty or not? He does not care whether it is a printed list, a notebook or a phone screen. An empty notebook means "nobody's coming", a notebook with even one name means "open the door". He also refuses entry to the word "nobody" written on a page — the page is not empty, so to him it is a name. Pytho

The problem

Question presented to candidate: "When you write if something: in Python, which values count as false? And how does Python decide that for an object of your own class?"

What a strong answer should cover:

The built-in falsy set: None, False, numeric zero of any type (0, 0.0, -0.0, 0j, Decimal(0), Fraction(0)), and empty containers ("", b"", (), [], {}, set(), frozenset(), range(0), bytearray()); everything else is truthy.
The protocol: bool(x) calls __bool__ if defined (it must return a real bool), otherwise __len__ (falsy when it is 0), otherwise the object is truthy.
Truthy surprises: "0", "False", " ", [0], [[]] and float("nan") are all truthy.
and / or short-circuit and return one of their operands, not True/False, which powers x or default.
The classic bug: if not x: treats a legitimate 0 or "" as missing; use if x is None: when absence and emptiness differ.

Code / implementation expected: Optional — a quick bool() table or a small class with __bool__ / __len__.

truthiness

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Beginners preparing for Python interviews — no prior Python knowledge assumed. Difficulty: Easy Python version: Behaviour checked on CPython 3.11 (python3.11 and the in-page Pyodide 0.25.1 runtime, which is CPython 3.11.3); one version-dependent case was checked on 3.12 and 3.13 as noted.

How to read this doc: Each idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact word an interviewer expects. Every output shown in the Verified section was copied from a real run of the examples on this page.

1. Why This Even Matters — A Story First

A doorman at a club checks one thing: is the guest list empty or not? He does not care whether it is a printed list, a notebook or a phone screen. An empty notebook means "nobody's coming", a notebook with even one name means "open the door". He also refuses

The solution is hidden so you can try first

9 min read · 9 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

PythonThe falsy family, truthy look-alikes, and and/or return values

Practice

PythonThe __bool__ / __len__ protocol and the 0-vs-None bug

Practice

Discussion

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