Python - Enums

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Enums in Python

In Python, 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. The Enum class included in enum module (which is a part of Python's standard library) is used as the parent class to define enumeration of a set of identifiers − conventionally written in upper case.

Example

In the below code, "subjects" is the enumeration. It has different enumeration members and each member is an object of the enumeration class subjects. These members have name and value attributes.

# importing enum 
from enum import Enum

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

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

It results in following output −

<enum 'subjects'> 

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

Example

In this example, we are using the @unique decorator to restrict duplicacy.

from enum import Enum, unique

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

This will raise an exception as shown below −

   @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 its constructor to create an enumeration. This constructor accepts two arguments, which are the name of enumeration and a string consisting of enumeration member symbolic names separated by a whitespace.

Example

following is an alternative method of defining an enumeration −

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

This code will give the following output −

subjects.ENGLISH
subjects.MATHS
subjects.SCIENCE
subjects.SANSKRIT

Accessing Modes in Enums

Members of an enum class can be accessed in two modes −

• Value − In this mode, value of the enum member is accessed using the "value" keyword followed by object of the enum class.

• Name − Similarly, we use the "name" keyword to access name of the enum member.

Example

The following example illustrates how to access value and name of the enum member.

from enum import Enum

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

It will produce the following output −

<enum 'subjects'> 
SANSKRIT
S

Iterating through Enums

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

Example

The following example shows how to iterate through an enumeration using for loop −

from enum import Enum

class subjects(Enum):
   ENGLISH = "E"
   MATHS = "M"
   GEOGRAPHY = "G"
   SANSKRIT = "S"

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

It will produce the following output −

ENGLISH E
MATHS M
GEOGRAPHY G
SANSKRIT S

We know that 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.