Question about ast.literal_eval

[Steven D'Aprano]

On Tue, 21 May 2013 08:30:03 +0200, Frank Millman wrote:

> On 20/05/2013 18:12, Steven D'Aprano wrote:

>> Personally, I would strongly suggest writing your own mini- evaluator
>> that walks the list and evaluates it by hand. It isn't as convenient as
>> just calling eval, but *definitely* safer.
>>
>>
> I am not sure I can wrap my mind around mixed 'and's, 'or's, and
> brackets.


Parsers are a solved problem in computer science, he says as if he had a 
clue what he was talking about *wink*

Here's a sketch of a solution... suppose you have a sequence of records, 
looking like this:

(bool_op, column_name, comparison_op, literal)

with appropriate validation on each field. The very first record has 
bool_op set to "or". Then, you do something like this:


import operator
OPERATORS = {
    '=': operator.eq,
    'is': operator.is_,
    '<': operator.lt,
    # etc.
    }


def eval_op(column_name, op, literal):
    value = lookup(column_name)  # whatever...
    return OPERATORS[op](value, literal)

result = False

for (bool_op, column_name, comparison_op, literal) in sequence:
    flag = eval_op(column_name, comparison_op, literal)
    if bool_op == 'and':
        result = result and flag
    else: 
        assert bool_op == 'or'
        result = result or flag
    # Lazy processing?
    if result:
        break


and in theory it should all Just Work.




-- 
Steven