C or VBA Enum

In C or VBA Enum, the value of each member may or may not be specified, otherwise it will be the value of the previous member + 1. The first member will be 0 if no value is specified.

Personally, when dealing with input files such as CSV in VBA, I often use Enum type because it is convenient to identify the field of the file. For example, it looks like the following.

Public Enum FieldID

     OrderNo = 1
     CustomerNo
     PurchaseDate
     ProductID = 10
     Price
     Units
     Amount

End Enum

As in this example, if you have a small number of elements, you may not appreciate it, but if you define a field in a file that has dozens of columns, all members will have columns added or removed. This is convenient because you do not have to renumber it.

In Python3.6, the function to automatically value the Enum type was added, so I tried to see if it could be done in the same way.

Python Enum

Python added the Enum class to the standard library in version 3.4. Python Enums are well documented in the here article.

As you can see in the Enum Library Documentation (https://docs.python.org/3/library/enum.html), Python 3.6 allows Enums to be automatically priced, = auto () The expression has been added. (Actually, ʻauto` is a class and calls its constructor.)

I tried the same thing as the VBA code above.

from enum import IntEnum, auto

class Field(IntEnum):

   OrderNo = 1
   CustomerNo = auto()
   PurchaseDate = auto()
   ProductID = 10
   Price = auto()
   Units = auto()
   Amount = auto()

for elm in Field:
   print(elm, int(elm))

The execution result is

Field.OrderNo 1
Field.CustomerNo 2
Field.PurchaseDate 3
Field.ProductID 10
Field.Price 11
Field.Units 12
Field.Amount 13

It is necessary to write ʻauto (), but it seems to increment automatically like C and VBA. However, if the first member is = auto ()`, it will be 1 instead of 0.

However, there is a note in the library documentation that this increment is implementation dependent. To make this behavior permanent, you need to define the behavior of ʻauto ()` yourself.

The value returned by ʻauto ()can be customized with thegenerate_next_value method, so create a class ʻAutoIntoEnum that inherits ʻIntEnum and define generate_next_value` yourself.

from enum import IntEnum, auto

class AutoIntEnum(IntEnum):

    def _generate_next_value_(name, start, count, last_values):
        for last_value in reversed(last_values):
            try:
                return last_value + 1
            except TypeError:
                pass
        else:
            return start

class Field(AutoIntEnum):

    OrderNo = auto()
    CustomerNo = auto()
    PurchaseDate = auto()
    ProductID = 10
    Price = auto()
    Units = auto()
    Amount = auto()

for elm in Field:
    print(elm, int(elm))

This is OK even if the implementation of _generate_next_value_ changes. The contents of _generate_next_value_ are copied as they are from Code of Enum class.

Try auto to automatically price Enums in Python 3.6

C or VBA Enum

Python Enum