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From: Peter Otten <__peter__@web.de>
Subject: Re: mutable ints: I think I have painted myself into a corner
Date: Sun, 19 May 2013 09:01:49 +0200
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Xref: csiph.com comp.lang.python:45554

Cameron Simpson wrote:

> TL;DR: I think I want to modify an int value "in place".
> 
> Yesterday I was thinking about various "flag set" objects I have
> floating around which are essentially bare "object"s whose attributes
> I access, for example:
> 
>   flags = object()
>   flags.this = True
>   flags.that = False
> 
> and then elsewhere:
> 
>   if flags.that:
>     do that ...
> 
> Nice and readable, but I thought to myself: so bulky!

Plus, it doesn't work:

>>> object().this = True
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'object' object has no attribute 'this'

> The use case for flags is essentially boolean/binary, and so a int
> accessed as a bitmask should be smaller.
> 
> So I pulled out my BitMask int subclass (which mostly transcribes
> the int as "A|B|C" for readability purposes, partly to dillute Nick
> Coglan's liking for bulky strings over compact ints on readability
> grounds:-), and gave the subclass attribute access.
> 
> This works just fine for querying the flags object, with code exactly
> like the "if" statement above.
> 
> But setting up a flags object? What I _want_ to write is code like this:
> 
>   Flags = BitMask('this', 'that')
> 
>   # set default state
>   flags = Flags()
>   flags.this = False
>   flags.that = True
>   ... iterate over some options ...: flags.this = True
> 
> and there's my problem. This would modify the int in place. There's
> no way to do that. For the base type (int) this makes perfect sense,
> as they're immutable.
> 
> Before I toss this approach and retreat to my former "object"
> technique, does anyone see a way forward to modify an int subclass
> instance in place? (That doesn't break math, preferably; I don't
> do arithmetic with these things but they are, after all, ints...)

No, but you could make

flags = Flags(this=False, that=True)

work.