What does __len__ do?
PICTURE THIS: HOW TO EXPLAIN IT
Simple meaning
__len__ is called by len() and should return a non-negative int.
WHY — Dunder Methods instead of guessing?
Why interviewers care about Dunder Methods:
people who only read docs from people who shipped.
and tied to Python work.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1__len__ is called by
len() and should return a non-negative int.
- 2Containers implement it so
they work with truthiness checks that use length.
- 3Returning something other than
an int raises TypeError.
- 4Give an example
One tiny concrete case you can say aloud.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
__len__ is called by len() and should return a non-negative int. Containers implement it so they work with truthiness checks that use length.