[Python-ideas] frozenset literals

[Yuriy Taraday]

On Sat, Feb 2, 2013 at 11:38 AM, Hua Lu <gotoalanlu at gmail.com> wrote:

> >>> 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>
>

It looks like we can special-case usage of set literal as a key for a dict
or a member for another set and create a frozenset constant instead.

So that adict[{1, 2, 3}] should be interpreted as adict[frozenset([1, 2,
3])] and { {'b', 'a', 'r'}, 'foo' } as { frozenset('bar'), 'foo' }

This will provide a minimal change to the interpreter while making it
possible to use any literal-parsing with frozensets while keeping method
calls out of literal_eval.

-- 

Kind regards, Yuriy.
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