Re: Making immutable instances

Ben Finney <bignose+hates-spam@xxxxxxxxxxxxxxx> writes:
> I'm looking for a "consenting adults" restriction: classes will have
> immutable instances only where it makes sense from the class protocol.
> I'm not going to lose sleep over users who go looking for trouble.

I think you're defining immutable differently than I do. Python
already supports attributes that clients can't rebind. You can make an
instance in which all attributes have that property. However
attributes added by a client aren't really yours. They don't change
the immutability of your attributes, or affect the behavior of your
class in any way. Such attributes don't really belong to your class;
they belong to the client.

Even in full B&D languages, it's trivial to overcome this restriction
with a subclass:

>>> i = int()
>>> i
0
>>> i.foo = 1
Traceback (most recent call last):
File "<stdin>", line 1, in ?
AttributeError: 'int' object has no attribute 'foo'
>>> class Aint(int): pass
....
>>> m = Aint()
>>> m
0
>>> m.foo = 1
>>> m.foo
1
>>>

The extra class is the kind of boilerplate that Python in general has
so little of, and B&D languages so much of. In Python, I can even fix
it so *your* code uses my wrapped version:

import Finney
class Addable(Finnney.Immutable): pass
Finney.Immutable = Addable

Which means that from now on *your* code that tries to create
Immutables will actually get Addables. The inability to do this in B&D
languages is - well, painfull. That Python doesns't require the
boilerplate in a good thing.

<mike
--
Mike Meyer <mwm@xxxxxxxxx> http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.
.

Relevant Pages

  • Re: Help understanding the decisions *behind* python? - immutable objects
    ... I've never heard a *good* reason why Python needs both. ... The good reason is the immutability, ... So things like "frozenset" had to be built into the language. ...
    (comp.lang.python)
  • Re: Help understanding the decisions *behind* python? - immutable objects
    ... I've never heard a *good* reason why Python needs both. ... The good reason is the immutability, ... So things like "frozenset" had to be built into the language. ...
    (comp.lang.python)
  • Re: Immutable Geometry Types
    ... I am learning python, having learnt most of my object orientation with ... used immutability in python before, so thought this would be an ... Suppress setting of data - Angle is immutable ...
    (comp.lang.python)
  • Re: Scope rule pecularities
    ... > Python has immutable types at all, including in the commonly used builtins. ... > Immutability of strings, numbers and tuples has worked very well for Python ... > happens inside func. ... >> writing code that can work with any sequence becomes a possible ...
    (comp.lang.python)
  • Re: f---ing typechecking
    ... as opposed to lists which are much ... so the immutability of their tupleness doesn't get in the way. ... I've used tuples internally in my Python code as ... cache keys for function memoizing. ...
    (comp.lang.python)