Replace FancyEqMixin and FancyStrMixin with something that better implements their intent

	@glyph reported
Trac ID	trac#6529
Type	enhancement
Created	2013-05-30 00:22:56Z

Despite being handy in many situations, FancyEqMixin and FancyStrMixin have a number of deficiencies:

• They don't support inheritance very well, because they don't aggregate compareAttributes and showAttributes across their inheritance hierarchy automatically
• You have to re-specify the list of "interesting" attributes twice, when it should really be the same in both cases; as well as likely being the same as at least part of the signature of __init__.
• FancyEqMixin implements equality and non-equality, but not ordering or hashing.
• They're old-style, so it's clearly time for a rewrite anyway.

What I think these classes are really trying to do is provide helpers for someone trying to implement a value - that is, a functionally immutable type, something like a named tuple, so that you can refer to the structured parts of a datum.

I think that we should provide a twisted.python.util.Value superclass that provides all of these semantics in a nice bundle; possibly still decomposed so you can have bits of it a la carte. Something like this:

#!py
class MyValue(ValueType(attributes='a b c'.split(), mutable=['a'])):
  def foo(self):
    return self.b + self.c

This would say: we have attributes ['a', 'b', 'c']. The presence of a mutable attribute, 'a', would disqualify MyValue from having a __hash__, but it could still be compared. We could also have other flags, like "comparable".

trac-id__6529 6529
type__enhancement enhancement
reporter__glyph glyph
priority__normal normal
milestone__ 
branch__ 
branch_author__ 
status__new new
resolution__None None
component__core core
keywords__ 
time__1369873376000000 1369873376000000
changetime__1560836483983902 1560836483983902
version__None None
owner__None None

Start by locating FancyEqMixin and FancyStrMixin in the Twisted codebase and reading their existing behavior. The issue proposes a twisted.python.util.Value abstraction with shared attributes and value semantics, including comparison and hashing rules; done would require an agreed design and implementation covering those stated concerns.