[Python-ideas] frozenset literals

[Antoine Pitrou]

On Sun, 3 Feb 2013 00:40:47 +1000
Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 3 Feb 2013 00:21, "Steven D'Aprano" <steve at pearwood.info> wrote:
> >
> > On 03/02/13 00:06, Stefan Behnel wrote:
> >>
> >> Hua Lu, 02.02.2013 08:38:
> >>>>>>
> >>>>>> import ast
> >>>>>> ast.literal_eval("{ 'key': 'val' }")
> >>>
> >>> {'key': 'val'}
> >>>>>>
> >>>>>> ast.literal_eval("{ ('key',): 'val' }")
> >>>
> >>> {('key',): 'val'}
> >>>>>>
> >>>>>> ast.literal_eval("{ frozenset({'key'}): 'val' }")
> >>>
> >>> Traceback (most recent call last):
> >>>    File "<stdin>", line 1, in<module>
> >>>    File "/usr/lib/python3.3/ast.py", line 86, in literal_eval
> >>>      return _convert(node_or_string)
> >>>    File "/usr/lib/python3.3/ast.py", line 63, in _convert
> >>>      in zip(node.keys, node.values))
> >>>    File "/usr/lib/python3.3/ast.py", line 62, in<genexpr>
> >>>      return dict((_convert(k), _convert(v)) for k, v
> >>>    File "/usr/lib/python3.3/ast.py", line 85, in _convert
> >>>      raise ValueError('malformed node or string: ' + repr(node))
> >>> ValueError: malformed node or string:<_ast.Call object at
> 0x7f865a8c1450>
> >>
> >>
> >> So, why exactly are you using ast.literal_eval() to evaluate a
> non-literal
> >> expression?
> >
> >
> >
> > Stefan, you did ask Hua Lu to show an example of what isn't working. It's
> just a demonstration of what doesn't work -- you can't create a frozenset
> using ast.literal_eval.
> >
> > I think Hua Lu's original post made it quite clear. He wishes there to be
> a frozenset literal, because currently there is no way to have
> ast.literal_eval evaluate something containing a frozenset. Because there
> is no frozenset literal.
> >
> > I think Raymond Hettinger's proposal back in 2008:
> >
> > http://mail.python.org/pipermail/python-3000/2008-January/011798.html
> >
> > and the following thread is worth reading. Guido even pronounced his
> agreement:
> >
> > http://mail.python.org/pipermail/python-3000/2008-January/011814.html
> >
> > but then changed his mind (as did Raymond). So the status quo remains.
> >
> > Unfortunately the proposal to use f{ ... } for frozen sets cannot work
> within the constraints of Python's lexer:
> >
> > http://mail.python.org/pipermail/python-3000/2008-January/011838.html
> 
> To clarify Guido's comment in that post, I'm fairly sure it *can* be made
> to work, it just won' t be the same way that string prefixes work (because
> the contents of dict and set displays are not opaque to the compiler the
> way string contents are).
> 
> The hypothetical "What if we want to allow expr{} as a general construct?"
> objection needs to be balanced against the immediate value of a more
> expressive language subset for use in ast.literal_eval().

I've just tried: ast.literal_eval() works just fine on a normal set:

>>> ast.literal_eval("{1,2,3}")
{1, 2, 3}

So I'd like to know why people find it important to build a frozenset
rather than a normal set. The situation of wanting to use sets as hash
keys is very rare in my experience.

Regards

Antoine.