Moderate Data Types Question 74 of 228

What goes wrong with a mutable default argument?

Python & Django · Speak this in 60–90 seconds · Faridabad & Delhi NCR

PICTURE THIS: PYTHON TYPES

intfloatstrboollisttupledictset

Simple meaning

The default object is created once, at function definition, and shared across calls.

WHY — Data Types instead of guessing?

Why interviewers care about Data Types:

Data Types questions separate

people who only read docs from people who shipped.

Keep it short, concrete,

and tied to Python work.

Stay structured

Name the idea, why it exists, then one short example.

Close cleanly

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
The default object is
created once, at function definition, and shared across calls.
2
A default list therefore
accumulates items unless you replace it.
3
The idiom is def
f(x=None): x = [] if x is None else x.
4
Give an example
One tiny concrete case you can say aloud.
5
Common mistake
What juniors usually get wrong.
6
Close
When you pick this over the alternative.

EXAMPLE — See it in action

Here's a short line you can speak, broken into clear beats:

Say this line

“A default list therefore accumulates items unless you replace it.”

Break into beats

Adefaultlistthereforeaccumulatesitems

Speaking order

2987408337471632900

Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.

Key takeaway

The default object is created once, at function definition, and shared across calls. A default list therefore accumulates items unless you replace it.