具有__index __（）方法的Python对象示例？

Question

What are some examples of Python objects that have an __index__() method other than an int ?

For example, the Hex documentation state:

If x is not a Python int object, it has to define an __index__() method that returns an integer.

This is for self-learning.

Answer 1

Mostly, these are types from mathematical libraries like NumPy or SymPy . These libraries have their own integer types (for good reason), but thanks to __index__ , their special integers can be used as list indices or passed to hex like normal integers.

In [9]: import sympy

In [10]: x = sympy.Integer(1)

In [11]: x  # It looks like a regular 1, but it's not.
Out[11]: 1

In [12]: x/2  # This object has special behavior that makes sense for SymPy...
Out[12]: 1/2

In [13]: [1, 2, 3][x]  # but you can still use it for things like indexing.
Out[13]: 2