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What is a decorator in Python, and how does it work under the hood?

For Data Scientist ML Engineer Data Engineer AI / LLM Engineer

The short answer

A decorator is a callable that takes a function, wraps it with extra behaviour, and returns the new callable. The @syntax is syntactic sugar for reassigning the function name to the wrapper immediately after definition.

How to think about it

What the interviewer wants to hear

They want to know you can explain decorators without just saying “it adds functionality to a function.” The key things to hit: it’s a callable that takes a function and returns a callable, the @ syntax is syntactic sugar for name-rebinding, and functools.wraps is essential. Then show you can build one from scratch.

The mechanism in plain terms

The @decorator syntax is exactly equivalent to calling the decorator manually:

def greet(name):
    print(f"Hello, {name}")

greet = decorator(greet)   # @decorator does this automatically

Python evaluates decorator(greet) right after the def, and binds the name greet to whatever is returned. If the decorator returns a wrapper function, greet now points to that wrapper — which closes over the original function.

Building a timing decorator step by step

Stacking decorators

Stacking applies them bottom-up — the decorator closest to the def runs first:

@timer
@repeat(times=3)
def fetch_data(url): ...
# Equivalent to: fetch_data = timer(repeat(times=3)(fetch_data))

Key insight

A decorator is just a higher-order function — a function that takes a function and returns a function. @ is purely syntactic sugar. Once you internalise that, you can reason about any decorator, including class-based decorators and decorators that accept arguments.

Learn it properly Decorators