[Python-Dev] constant/enum type in stdlib

[Michael Foord]

On 28/11/2010 17:05, Michael Foord wrote:
> [snip...]
> It is sensible if flag values are only powers of 2; we could enforce 
> that or not... (Another one for the optional feature list.)
>
Another 'optional' feature I omitted was Phillip J. Eby's suggestion / 
requirement that named values be pickleable. Email is clunky for 
handling this, is there enough support (there is still some objection 
that is sure) to revive the PEP or create a new one?

I also didn't include Nick's suggested API, which is slightly different 
from the one I suggested:

silly = Namegroup.from_names("Silly", "FOO", "BAR", "BAZ")
 >>> silly.FOO
Silly.FOO=0
 >>> int(silly.FOO)
0
 >>> silly(0)
Silly.FOO=0

x = named_value("FOO", 1)
y = named_value("BAR", "Hello World!")
z = named_value("BAZ", dict(a=1, b=2, c=3))

set_named_values(globals(), foo=x._raw(), bar=y._raw(), baz=z._raw())

Where a named value created from an integer is an int subclass, from a 
dict a dict subclass and so on.

Michael



> I forgot auto-enumeration (specifying names only and having values 
> autogenerated) from the optional feature set by the way. I think 
> Antoine strongly disapproves of this feature because it reminds him of 
> C enums.
>
> Mark Dickinson thinks that the flags feature could be an optional 
> feature too. If we have ORing it makes sense to have ANDing, so I 
> guess they belong together. I think there is value in it though.
>
> I realise that the optional feature list is now not small, and 
> implementing all of it would create the "franken-api" Nick is worried 
> about. The minimal feature list is nicely small though and provides 
> useful functionality.
>
> All the best,
>
> Michael
>
>>
>> Grouped constants
>> ----------------
>> * Easy to create a group of named constants, accessible as attributes 
>> on group object
>> * Capability to go from name or value to corresponding constants
>>
>>
>> Optional Features
>> ---------------
>>
>> * Ability to dynamically add new named values to a group. (Suggested 
>> by Guido)
>> * Ability to test if a name or value is in a group
>> * Ability to list all names in a group
>> * ANDing as well as ORing
>> * Constants are unique
>> * OR'ing with an integer will look up the name (or calculate it if 
>> the int itself represents flags that have already been OR'd) and 
>> return a named value (with useful repr) instead of just an integer
>> * Named constants be named values that can wrap *any* type and not 
>> just immutable values. (Note that wrapping mutable types makes 
>> providing "from_value" functionality harder *unless* we guarantee 
>> that named values are unique. If they aren't unique named values for 
>> a mutable type can have different values and there is no single 
>> definition of what the named value actually is.)
>> Requiring that values only have one name - or alternatively that 
>> values on a group could have multiple names (obviously incompatible 
>> features).
>> * Requiring all names in a group to be of the same type
>> * Allow names to be set automatically in a namespace, for example in 
>> a class namespace or on a module
>> * Allow subclassing and adding of new values only present in subclass
>>
>>
>> I'd rather we agree a suitable (minimal) API and feature set and go 
>> to implementation from that.
>>
>> For wrapping mutable types I'm tempted to say YAGNI. For the standard 
>> library wrapping integers meets almost all our use-cases except for 
>> one float. (At work we have a decimal constant as it happens.) 
>> Perhaps we could require immutable types for groups but allow 
>> arbitrary values for individual named values?
>>
>> For the named values api:
>>
>> name = NamedValue('name', value)
>>
>> For the grouping (tentatively accepted as reasonable by Antoine):
>>
>> Group = make_constants('Group', name1=value1, name2=value2)
>> name1, name2 = Group.name1, Group.name1
>> flag = name1 | name2
>>
>> value = int(Group.name1)
>> name = Group('name1')
>> # alternatively: value = Group.from_name('name1')
>> name = Group.from_value(value1)
>> # Group(value1) could work only if values aren't strings
>> # perhaps: name = Group(value=value1)
>>
>> Group.new_name = value3 # create new value on the group
>> names = Group.all_names()
>> # further bikeshedding on spelling of all_names required
>> # correspondingly 'all_values' I guess, returning the constants 
>> themselves
>>
>> Some of the optional features couldn't later be added without 
>> backwards compatibility concerns (I think the type checking features 
>> and requiring unique values for example). We should at least consider 
>> these if we are to make adding them later difficult. I would be fine 
>> with not having these features.
>>
>> All the best,
>>
>> Michael
>>
>>> Cheers,
>>> Nick.
>>>
>>
>>
>
>


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