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Date: Sat, 6 Jun 2015 15:29:08 +1000
Subject: Re: Can Python function return multiple data?
From: Chris Angelico <rosuav@gmail.com>
Cc: "python-list@python.org" <python-list@python.org>
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On Sat, Jun 6, 2015 at 2:50 PM, Steven D'Aprano <steve@pearwood.info> wrote:
> This does not happen:
>
>     mylist = []
>     mytuple = (None, 1, mylist)
>     mylist.append(0)
>     => raises an exception
>
> The *tuple* is immutable, not the list.

What you could have is a "FrozenList" (by analogy with frozenset),
something like this:

class FrozenList(tuple):
    def __new__(*a, **kw):
        self = tuple.__new__(*a, **kw)
        hash(self) # If error, disallow construction
        return self

That would raise the error at the moment of mytuple's creation, not at
mylist's mutation, but that's about as close as I can think of to a
"this truly must be immutable" object in Python. And of course, any
object can lie (or be mistaken) about its hashability and mutability:

>>> def adder(x, inc=1): return x+inc
...
>>> hash(adder)
-9223363254790714706
>>> FrozenList((adder,))
(<function adder at 0x7fcbbcf0aae8>,)
>>> descr = {adder: "Add 1 to a number"}

Looks immutable to me.

>>> adder.__defaults__=2,
>>> next(iter(descr))(6)
8

Oops.

ChrisA