[Python-Dev] constant/enum type in stdlib

[Michael Foord]

On 25/11/2010 09:34, Glenn Linderman wrote:
> So the following code defines constants with associated names that get 
> put in the repr.
>
> I'm still a Python newbie in some areas, particularly classes and 
> metaclasses, maybe more.
> But this Python 3 code seems to create constants with names ... works 
> for int and str at least.
>
> Special case for int defines a special  __or__ operator to OR both the 
> values and the names, which some might like.
>
> Dunno why it doesn't work for dict, and it is too late to research 
> that today.  That's the last test case in the code below, so you can 
> see how it works for int and string before it bombs.
>
> There's some obvious cleanup work to be done, and it would be nice to 
> make the names actually be constant... but they do lose their .name if 
> you ignorantly assign the base type, so at least it is hard to change 
> the value and keep the associated .name that gets reported by repr, 
> which might reduce some confusion at debug time.
>
> An idea I had, but have no idea how to implement, is that it might be 
> nice to say:
>
>     with imported_constants_from_module:
>            do_stuff
>
> where do_stuff could reference the constants without qualifying them 
> by module.  Of course, if you knew it was just a module of constants, 
> you could "import * from module" :)  But the idea of with is that 
> they'd go away at the end of that scope.
>
> Some techniques here came from Raymond's namedtuple code.
>
>
> def constant( name, val ):
>     typ = str( type( val ))
>     if typ.startswith("<class '")  and  typ[ -2: ] == "'>":
>         typ = typ[ 8:-2 ]
>     ev = '''
> class constant_%s( %s ):
>     def __new__( cls, val, name ):
>         self = %s.__new__( cls, val )
>         self.name = name
>         return self
>     def __repr__( self ):
>         return self.name + ': ' + str( self )
> '''
>     if typ == 'int':
>         ev += '''
>     def __or__( self, other ):
>         if isinstance( other, constant_int ):
>             return constant_int( int( self ) | int( other ),
>                                  self.name + ' | ' + other.name )
> '''

Not quite correct. If you or a value you with itself you should get back 
just the value not something with "name|name" as the repr.

We can hold off on implementations until we have general agreement that 
some kind of named constant *should* be added, and what the feature set 
should look like.

All the best,

Michael

>     ev += '''
> %s = constant_%s( %s, '%s' )
>
> '''
>     ev = ev % ( typ, typ, typ, name, typ, repr( val ), name )
>     print( ev )
>     exec( ev, globals())
>
> constant('O_RANDOM', val=16 )
>
> constant('O_SEQUENTIAL', val=32 )
>
> constant("O_STRING", val="string")
>
> def foo( x ):
>     print( str( x ))
>     print( repr( x ))
>     print( type( x ))
>
> foo( O_RANDOM )
> foo( O_SEQUENTIAL )
> foo( O_STRING )
>
> zz = O_RANDOM | O_SEQUENTIAL
>
> foo( zz )
>
> y = {'ab': 2, 'yz': 3 }
> constant('O_DICT', y )
>
>
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