Lazily-evaluated Property Pattern in Python

Lazy evaluation is a useful pattern that can improve your code's efficiency in many situations.

One example of this is instance attributes that take long to compute:

This approach may cause initialization to take unnecessarily long, especially when you don't always need to access Person#relatives.

A better strategy would be to get relatives only when it's needed (i.e., "lazily"):

In this case, the list of relatives is only computed the first time Person#relatives is accessed. After that, it's stored in Person#_relatives to prevent repeated evaluations.

A perhaps more Pythonic approach would be to use a decorator that makes a property lazy-evaluated

This removes a lot of boilerplate, especially when an object has many lazily-evaluated properties.