[Python-ideas] frozenset literals

[Steven D'Aprano]

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

Unfortunately we're running out of useful, easy to enter symbols for literals. Until such time (Python4000 perhaps, or more likely Python5000) as we can use a rich set of Unicode literals, I don't think there is any clear way to have a frozenset literal.



-- 
Steven