When should I subclass EnumMeta instead of Enum?
The best (and only) cases I have seen so far for subclassing EnumMeta
comes from these four questions:
A more pythonic way to define an enum with dynamic members
Prevent invalid enum attribute assignment
Create an abstract Enum class
Invoke a function when an enum member is accessed
We'll examine the dynamic member case further here.
First, a look at the code needed when not subclassing EnumMeta
:
The stdlib way
from enum import Enum
import json
class BaseCountry(Enum):
def __new__(cls, record):
member = object.__new__(cls)
member.country_name = record['name']
member.code = int(record['country-code'])
member.abbr = record['alpha-2']
member._value_ = member.abbr, member.code, member.country_name
if not hasattr(cls, '_choices'):
cls._choices = {}
cls._choices[member.code] = member.country_name
cls._choices[member.abbr] = member.country_name
return member
def __str__(self):
return self.country_name
Country = BaseCountry(
'Country',
[(rec['alpha-2'], rec) for rec in json.load(open('slim-2.json'))],
)
The aenum
way 12
from aenum import Enum, MultiValue
import json
class Country(Enum, init='abbr code country_name', settings=MultiValue):
_ignore_ = 'country this' # do not add these names as members
# create members
this = vars()
for country in json.load(open('slim-2.json')):
this[country['alpha-2']] = (
country['alpha-2'],
int(country['country-code']),
country['name'],
)
# have str() print just the country name
def __str__(self):
return self.country_name
The above code is fine for a one-off enumeration -- but what if creating Enums from JSON files was common for you? Imagine if you could do this instead:
class Country(JSONEnum):
_init_ = 'abbr code country_name' # remove if not using aenum
_file = 'some_file.json'
_name = 'alpha-2'
_value = {
1: ('alpha-2', None),
2: ('country-code', lambda c: int(c)),
3: ('name', None),
}
As you can see:
-
_file
is the name of the json file to use -
_name
is the path to whatever should be used for the name -
_value
is a dictionary mapping paths to values3 -
_init_
specifies the attribute names for the different value components (if usingaenum
)
The JSON data is taken from https://github.com/lukes/ISO-3166-Countries-with-Regional-Codes -- here is a short excerpt:
[{"name":"Afghanistan","alpha-2":"AF","country-code":"004"},
{"name":"Åland Islands","alpha-2":"AX","country-code":"248"},
{"name":"Albania","alpha-2":"AL","country-code":"008"},
{"name":"Algeria","alpha-2":"DZ","country-code":"012"}]
Here is the JSONEnumMeta
class:
class JSONEnumMeta(EnumMeta):
@classmethod
def __prepare__(metacls, cls, bases, **kwds):
# return a standard dictionary for the initial processing
return {}
def __init__(cls, *args , **kwds):
super(JSONEnumMeta, cls).__init__(*args)
def __new__(metacls, cls, bases, clsdict, **kwds):
import json
members = []
missing = [
name
for name in ('_file', '_name', '_value')
if name not in clsdict
]
if len(missing) in (1, 2):
# all three must be present or absent
raise TypeError('missing required settings: %r' % (missing, ))
if not missing:
# process
name_spec = clsdict.pop('_name')
if not isinstance(name_spec, (tuple, list)):
name_spec = (name_spec, )
value_spec = clsdict.pop('_value')
file = clsdict.pop('_file')
with open(file) as f:
json_data = json.load(f)
for data in json_data:
values = []
name = data[name_spec[0]]
for piece in name_spec[1:]:
name = name[piece]
for order, (value_path, func) in sorted(value_spec.items()):
if not isinstance(value_path, (list, tuple)):
value_path = (value_path, )
value = data[value_path[0]]
for piece in value_path[1:]:
value = value[piece]
if func is not None:
value = func(value)
values.append(value)
values = tuple(values)
members.append(
(name, values)
)
# get the real EnumDict
enum_dict = super(JSONEnumMeta, metacls).__prepare__(cls, bases, **kwds)
# transfer the original dict content, _items first
items = list(clsdict.items())
items.sort(key=lambda p: (0 if p[0][0] == '_' else 1, p))
for name, value in items:
enum_dict[name] = value
# add the members
for name, value in members:
enum_dict[name] = value
return super(JSONEnumMeta, metacls).__new__(metacls, cls, bases, enum_dict, **kwds)
# for use with both Python 2/3
JSONEnum = JSONEnumMeta('JsonEnum', (Enum, ), {})
A few notes:
JSONEnumMeta.__prepare__
returns a normaldict
EnumMeta.__prepare__
is used to get an instance of_EnumDict
-- this is the proper way to get onekeys with a leading underscore are passed to the real
_EnumDict
first as they may be needed when processing the enum membersEnum members are in the same order as they were in the file
1 Disclosure: I am the author of the Python stdlib Enum
, the enum34
backport, and the Advanced Enumeration (aenum
) library.
2 This requires aenum 2.0.5+
.
3 The keys are numeric to keep multiple values in order should your Enum
need more than one.