Yet another attempt at a safe eval() call

[Ken Seehart]

On 1/4/2013 5:33 AM, Steven D'Aprano wrote:
> On Fri, 04 Jan 2013 07:24:04 -0500, Terry Reedy wrote:
>
>> On 1/3/2013 6:25 PM, Grant Edwards wrote:
>>> I've written a small assembler in Python 2.[67], and it needs to
>>> evaluate integer-valued arithmetic expressions in the context of a
>>> symbol table that defines integer values for a set of names.  The
>>> "right" thing is probably an expression parser/evaluator using ast, but
>>> it looked like that would take more code that the rest of the assembler
>>> combined, and I've got other higher-priority tasks to get back to.
>> Will ast.literal_eval do what you want?
> No. Grant needs to support variables, not just literal constants, hence 
> the symbol table.
>
>
Apologies for the delayed response...

Seems like it would be a bit safer and easier to approach this problem
by stretching the capability of ast.literal_eval() rather than
attempting to sandbox eval().

How about ast.literal_eval after performing lexical substitution using
the symbol table?

Assignment into the symbol table, and error handling, are exercises left
to the reader.

Something vaguely like this:

    /pseudocode:/

    def safe_eval(s, symbols={}):
        while search(s, r'\w+'):
            replace match with '('+repr(symbols[match])+')' in s
        return ast.literal_eval(s)

- Ken

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