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Extended Enum classes for the Python 3 enum module

Project description

extenum

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Extended Enum classes for the Python 3 enum module.

The enum module was added since 3.4. That’s good enough for simple use. The extenum is strongly inspired by Java Enum style described in Effective Java and privides additional feature.

How to install

NOTE: extenum supports Python 3 only.

$ pip install extenum

ConstantSpecificEnum

ConstantSpecificEnum class is inherited the standard Enum class and provides the feature of constant specific method and function overloading for Enum members.

Read Effective Java for more detail.

Constant specific method implementation

Let’s try to create Enum class with ConstantSpecificEnum. To use method as function overloading, create the registory with RegisterFactory for target Enum class.

>>> from extenum import ConstantSpecificEnum, RegisterFactory
>>> class Operation(ConstantSpecificEnum):
...     PLUS = '+'
...     MINUS = '-'
...     TIMES = '*'
...     DIVIDE = '/'
...
...     overload = RegisterFactory()
...
...     @overload.register(PLUS)
...     def apply(self, x, y):
...         return x + y
...
...     @overload.register(MINUS)
...     def apply(self, x, y):
...         return x - y
...
...     @overload.register(TIMES)
...     def apply(self, x, y):
...         return x * y
...
...     @overload.register(DIVIDE)
...     def apply(self, x, y):
...         return x / y
...
>>> for name, const in Operation.__members__.items():
...     print(name, ':', const.apply(2, 4))
...
PLUS : 6
MINUS : -2
TIMES : 8
DIVIDE : 0.5

Strategy enum pattern

The strategy enum is more complex pattern based on constant specific method.

>>> from extenum import ConstantSpecificEnum, RegisterFactory
>>> class PayrollDay(ConstantSpecificEnum):
...
...     class PayType(ConstantSpecificEnum):
...         WEEKDAY = 1
...         WEEKEND = 2
...
...         overload = RegisterFactory()
...
...         @overload.register(WEEKDAY)
...         def overtime_pay(self, hours, pay_rate):
...             return 0 if hours <= 8 else (hours - 8) * pay_rate / 2
...
...         @overload.register(WEEKEND)
...         def overtime_pay(self, hours, pay_rate):
...             return hours * pay_rate / 2
...
...         def pay(self, hours_worked, pay_rate):
...             base_pay = hours_worked * pay_rate
...             overtime_pay = self.overtime_pay(hours_worked, pay_rate)
...             return base_pay + overtime_pay
...
...     MONDAY = PayType.WEEKDAY
...     TUESDAY = PayType.WEEKDAY
...     WEDNESDAY = PayType.WEEKDAY
...     THURSDAY = PayType.WEEKDAY
...     FRIDAY = PayType.WEEKDAY
...     SATURDAY = PayType.WEEKEND
...     SUNDAY = PayType.WEEKEND
...
...     def pay(self, hours_worked, pay_rate):
...         return self.value.pay(hours_worked, pay_rate)
...
>>> PayrollDay.MONDAY.pay(10, 1000.0)
11000.0
>>> PayrollDay.WEDNESDAY.pay(8, 1000.0)
8000.0
>>> PayrollDay.SATURDAY.pay(10, 1000.0)
15000.0
>>> PayrollDay.SUNDAY.pay(8, 1000.0)
12000.0

ImplicitEnum

Before describing what ImplicitEnum class is, read good article written by Nick Coghlan as below.

OK. I guess you’ve already understood why the standard enum module haven’t supported implicit declaration syntax.

Put aside its needs for now, Nick indicates how to implement ImplicitEnum. So, let’s try to implement it experimentally using the special method, __missing__ in defaultdict and __prepare__ in Metaclass.

>>> from extenum import ImplicitEnum
>>> class Color(ImplicitEnum):
...     RED
...     GREEN
...     BLUE
...
>>> for name, const in Color.__members__.items():
...     print(name, ':', const.value)
...
RED : 1
GREEN : 2
BLUE : 3

It works well if some constants are explicit and the rest are implicit.

>>> class Numbers(ImplicitEnum):
...     ONE = 1
...     TWO = 2
...     THREE
...
>>> Numbers.THREE.value
3

However, it depends on the declation order.

>>> class DuplicatedValues(ImplicitEnum):
...     ONE
...     TWO = 1
...     THREE = 1
...
>>> DuplicatedValues.ONE.value
1
>>> DuplicatedValues.TWO.value
1
>>> DuplicatedValues.THREE.value
1

ChangeLog

0.6.0 (2015-03-05)

  • added ImplicitEnum

0.5.0 (2015-03-01)

  • first release

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