Python - Enums

The term 'enumeration' refers to the process of assigning fixed constant values to a set of strings, so that each string can be identified by the value bound to it. Python's standard library offers the enum module. The Enum class included in the enum module is used as the parent class to define the enumeration of a set of identifiers − conventionally written in upper case.

Enum Example

from enum import Enum

class subjects(Enum):
   ENGLISH = 1
   MATHS = 2
   SCIENCE = 3
   SANSKRIT = 4

In the above code, "subjects" is the enumeration. It has different enumeration members, e.g., subjects.MATHS. Each member is assigned a value.

Each member is ab object of the enumeration class subjects, and has name and value attributes.

obj = subjects.MATHS
print (type(obj), obj.value)

It results in following output −

<enum 'subjects'> 2

String-based Enum

Value bound to the enum member needn't always be an integer, it can be a string as well. See the following example −

Example

from enum import Enum

class subjects(Enum):
   ENGLISH = "E"
   MATHS = "M"
   GEOGRAPHY = "G"
   SANSKRIT = "S"
   
obj = subjects.SANSKRIT
print (type(obj), obj.name, obj.value)

It will produce the following output −

<enum 'subjects'> SANSKRIT S

Iterating Through Enum Members

You can iterate through the enum members in the order of their appearance in the definition, with the help of a for loop −

Example

for sub in subjects:
   print (sub.name, sub.value)

It will produce the following output −

ENGLISH E
MATHS M
GEOGRAPHY G
SANSKRIT S

The enum member can be accessed with the unique value assigned to it, or by its name attribute. Hence, subjects("E") as well as subjects["ENGLISH"] returns subjects.ENGLISH member.

Enum with @unique Decorator

An enum class cannot have same member appearing twice, however, more than one members may be assigned same value. To ensure that each member has a unique value bound to it, use the @unique decorator.

Example

from enum import Enum, unique

@unique
class subjects(Enum):
   ENGLISH = 1
   MATHS = 2
   GEOGRAPHY = 3
   SANSKRIT = 2

This will raise an exception as follows −

   @unique
    ^^^^^^
   raise ValueError('duplicate values found in %r: %s' %
ValueError: duplicate values found in <enum 'subjects'>: SANSKRIT -> MATHS

The Enum class is a callable class, hence you can use the following alternative method of defining enumeration −

from enum import Enum
subjects = Enum("subjects", "ENGLISH MATHS SCIENCE SANSKRIT")

The Enum constructor uses two arguments here. First one is the name of enumeration. Second argument is a string consisting of enumeration member symbolic names, separated by a whitespace.

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