What does the @unique decorator do in python?

Question

I recently came across this code:

@unique
class NetlistKind(IntEnum):
  Unknown = 0
  LatticeNetlist = 1
  QuartusNetlist = 2
  XSTNetlist = 4
  CoreGenNetlist = 8
  All = 15

What does the @unique decorator do, and what is its purpose in the above snippet?

Answer 1

Summary

unique is a class decorator for Enum that raises a ValueError if there are any duplicate enumeration values.

This code

from enum import unique, Enum

@unique
class Mistake(Enum):
    ONE = 1
    TWO = 2
    THREE = 3
    FOUR = 3

Produces this error:

ValueError: duplicate values found in <enum 'Mistake'>: FOUR -> THREE

More Details

From the documentation:

Context

By default, enumerations allow multiple names as aliases for the same value. When this behavior isn’t desired, [unique] can be used to ensure each value is used only once in the enumeration

Description

[unique is] a class decorator specifically for enumerations. It ensures only one name is bound to any one value [in an enum]. [unique] searches an enumeration’s __members__ gathering any aliases it finds; if any are found ValueError is raised with the details