On 21/05/2013 09:21, Steven D'Aprano wrote:
On Tue, 21 May 2013 08:30:03 +0200, Frank Millman wrote:
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.
That's very clever - thanks, Steven.
It doesn't address the issue of brackets. I imagine that the answer is
something like -
maintain a stack of results
for each left bracket, push a level
for each right bracket, pop the result
or something ...
I am sure that with enough trial and error I can get it working, but I
might cheat for now and use the trick I mentioned earlier of calling
eval() on a sequence of manually derived True/False values. I really
can't see anything going wrong with that.
BTW, thanks to ChrisA for the following tip -
import operator
ops = {
'in':lambda x,y: x in y, # operator.contains has the args backwards
I would have battled with that one.
Frank
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