[Python-Dev] PEP 435 - ref impl disc 2

Glenn Linderman v+python at g.nevcal.com
Sat May 11 07:15:59 CEST 2013

So, thanks everyone for helping me understand the metaclass issues, and 
helping fix my code and the reference implementation, so that I got a 
working workaround for enumerations.

Twiddling some more.... newly using hg and bitbucket... learned a lot 
today... at <https://bitbucket.org/v_python/ref435a/src>

Premise: For API parameters that are bitfields, it would be nice to have 
an Enum-type class that tracks the calculations done with the named 
values to create bit masks.

Currently available: Enum (or IntEnum) that can group the collection of 
named bit-field values into values of a single and unique type, but 
loses the names during calculations.

Just written: a class IntET (for "int expression tracker" which has an 
expression name as well as a value.  As a side effect, this could be 
used to track effective calculations of integers for debugging, because 
that is the general mechanism needed to track combined flag values, also 
for debug reporting.

So it is quite possible to marry the two, as Ethan helped me figure out 
using an earlier NamedInt class:

class NIE( IntET, Enum ):
     x = ('NIE.x', 1)
     y = ('NIE.y', 2)
     z = ('NIE.z', 4)

and then expressions involving members of NIE (and even associated 
integers) will be tracked... see demo1.py.

But the last few lines of demo1 demonstrate that NIE doesn't like, 
somehow, remember that its values, deep down under the covers, are 
really int.  And doesn't even like them when they are wrapped into IntET 
objects.  This may or may not be a bug in the current Enum implementation.

It is cumbersome to specify redundant names for the enumeration members 
and the underlying IntET separately, however.

Turns out that adding one line to ref435 (which I did in ref435a) will 
allow (nay, require) that base types for these modified Enums must have 
names, which must be supplied as the first parameter to their 
constructor.  This also works around whatever problem "real" Enum has 
with using named items internally, as demonstrated by demo2.py 
(reproduced in part here):

class NIE( IntET, Enum ):
     x = 1
     y = 2
     z = 4

print( repr( NIE.x + NIE.y ))
IntET('(NIE.x + NIE.y)', 3)

So the questions are:
1) Is there a bug in ref435 Enum that makes demo1 report errors instead 
of those lines working?
2) Is something like demo2 interesting to anyone but me? Of course, I 
think it would be great for reporting flag values using names rather 
than a number representing combined bit fields.
3) I don't see a way to subclass the ref435 EnumMeta except by replacing 
the whole __new__ method... does this mechanism warrant a slight 
refactoring of EnumMeta to make this mechanism easier to subclass with 
less code redundancy?
4) Or is it simple enough and useful enough to somehow make it a feature 
of EnumMeta, enabled by a keyword parameter?  Or one _could_ detect the 
existence of a __name__ property on the first base type, and key off of 
that, but that may sometimes be surprising (of course, that is what 
documentation is for: to explain away the surprises people get when they 
don't read it).
5) All this is based on "IntET"... which likely suffices for API flags 
parameters... but when I got to __truediv__ and __rtruediv__, which 
don't return int, then I started wondering how to write a vanilla ET 
class that inherits from "number" instead of "int" or "float"? One 
could, of course, make cooperating classes FloatET and DecimalET .... is 
this a language limitation, or is there more documentation I haven't 
read? :)   (I did read footnote [1] of 
and trembled.)

Probably some of these questions should be on stackoverflow or python 
ideas, but it is certainly an outgrowth of the Enum PEP, and personally, 
I'd hate to see flag APIs converted to Enum without the ability to track 
combinations of them... so I hope that justifies parts of this 
discussion continuing here. I'm happy to take pieces to other places, if 
so directed.
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