Using Enum Classes | Ucode

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Using Enum Classes ​

When we need to define constants, one way is to use uppercase variables defined with integers. For example, months:

python
JAN = 1
FEB = 2
MAR = 3
...
NOV = 11
DEC = 12

The advantage is simplicity, but the drawback is that the type is int, and they are still variables.

A better approach is to define a class type for such enumeration types, where each constant is a unique instance of the class. Python provides the Enum class to achieve this functionality:

python
from enum import Enum

Month = Enum('Month', ('Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'))

This way, we obtain an enumeration class of type Month. You can directly reference a constant using Month.Jan or iterate through all members:

python
for name, member in Month.__members__.items():
    print(name, '=>', member, ',', member.value)

The value attribute is an automatically assigned int constant, starting from 1 by default.

If you need more precise control over the enumeration type, you can derive a custom class from Enum:

python
from enum import Enum, unique

@unique
class Weekday(Enum):
    Sun = 0  # The value of Sun is set to 0
    Mon = 1
    Tue = 2
    Wed = 3
    Thu = 4
    Fri = 5
    Sat = 6

The @unique decorator helps ensure that there are no duplicate values.

There are several ways to access these enumeration types:

python
>>> day1 = Weekday.Mon
>>> print(day1)
Weekday.Mon
>>> print(Weekday.Tue)
Weekday.Tue
>>> print(Weekday['Tue'])
Weekday.Tue
>>> print(Weekday.Tue.value)
2
>>> print(day1 == Weekday.Mon)
True
>>> print(day1 == Weekday.Tue)
False
>>> print(Weekday(1))
Weekday.Mon
>>> print(day1 == Weekday(1))
True
>>> Weekday(7)
Traceback (most recent call last):
  ...
ValueError: 7 is not a valid Weekday
>>> for name, member in Weekday.__members__.items():
...     print(name, '=>', member)
...
Sun => Weekday.Sun
Mon => Weekday.Mon
Tue => Weekday.Tue
Wed => Weekday.Wed
Thu => Weekday.Thu
Fri => Weekday.Fri
Sat => Weekday.Sat

As seen, you can reference enumeration constants by their member names or directly obtain enumeration constants based on their value.

Exercise ​

Modify the gender attribute of the Student class to use an enumeration type, which helps avoid using strings:

python
from enum import Enum, unique

class Gender(Enum):
    Male = 0
    Female = 1

class Student(object):
    def __init__(self, name, gender):
        self.name = name
        self.gender = gender

# Test:
bart = Student('Bart', Gender.Male)
if bart.gender == Gender.Male:
    print('Test passed!')
else:
    print('Test failed!')

Summary ​

Enum allows you to define a set of related constants within a class. The class is immutable, and its members can be compared directly.