On 21/05/2013 09:23, Fábio Santos wrote:
On 21 May 2013 09:10, "Frank Millman" mailto:fr...@chagford.com>> wrote:
> 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
On 21 May 2013 09:10, "Frank Millman" wrote:
> 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 ...
>
Time for me to suggest
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'
On Tue, May 21, 2013 at 4:46 PM, Frank Millman wrote:
> You may be right, Chris, but I don't think my approach is all that bad.
Frankly, I'm not altogether convinced that our approach is right
either :) But like the Oracle in the Matrix, I'm not here to push you
to one decision or another so much
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.
On 20/05/2013 18:13, Chris Angelico wrote:
On Mon, May 20, 2013 at 11:26 PM, Frank Millman wrote:
0 - for the first entry in the list, the word 'check' (a placeholder - it is
discarded at evaluation time), for any subsequent entries the word 'and' or
'or'.
1 - left bracket - either '(' or ''.
On 20/05/2013 18:12, Steven D'Aprano wrote:
On Mon, 20 May 2013 15:26:02 +0200, Frank Millman wrote:
Can anyone see anything wrong with the following approach. I have not
definitely decided to do it this way, but I have been experimenting and
it seems to work.
[...]
It seems safe to me too,
On 21/05/2013 04:39, matt.newvi...@gmail.com wrote:
You might find the asteval module (https://pypi.python.org/pypi/asteval) useful. It
provides a relatively safe "eval", for example:
>>> import asteval
>>> a = asteval.Interpreter()
>>> a.eval('x = "abc"')
>>> a.eval('x i
On Monday, May 20, 2013 2:05:48 AM UTC-5, Frank Millman wrote:
> Hi all
>
>
>
> I am trying to emulate a SQL check constraint in Python. Quoting from
>
> the PostgreSQL docs, "A check constraint is the most generic constraint
>
> type. It allows you to specify that the value in a certain column mus
On Tue, May 21, 2013 at 2:12 AM, 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.
Probably faster, too, for what it's worth - eval i
On Mon, May 20, 2013 at 11:26 PM, Frank Millman wrote:
> 0 - for the first entry in the list, the word 'check' (a placeholder - it is
> discarded at evaluation time), for any subsequent entries the word 'and' or
> 'or'.
>
> 1 - left bracket - either '(' or ''.
>
> 5 - right bracket - either ')' or
On Mon, 20 May 2013 15:26:02 +0200, Frank Millman wrote:
> Can anyone see anything wrong with the following approach. I have not
> definitely decided to do it this way, but I have been experimenting and
> it seems to work.
>
> I store the boolean test as a json'd list of 6-part tuples. Each eleme
On 20/05/2013 10:07, Frank Millman wrote:
On 20/05/2013 09:55, Chris Angelico wrote:
Is it a requirement that they be able to key in a constraint as a
single string? We have a similar situation in one of the systems at
work, so we divided the input into three(ish) parts: pick a field,
pick an op
On 20 May 2013 09:19, "Frank Millman" wrote:
> Quoting from the manual -
>
> "Safely evaluate an expression node or a string containing a Python
expression. The string or node provided may only consist of the following
Python literal structures: strings, bytes, numbers, tuples, lists, dicts,
sets,
On Mon, 20 May 2013 10:55:35 +0300, Carlos Nepomuceno wrote:
> I understand your motivation but I don't know what protection
> ast.literal_eval() is offering that eval() doesn't.
eval will evaluate any legal Python expression:
py> eval("__import__('os').system('echo Mwahaha! Now you are pwned!'
On 20/05/2013 09:55, Carlos Nepomuceno wrote:
Why don't you use eval()?
Because users can create their own columns, with their own constraints.
Therefore the string is user-modifiable, so it cannot be trusted.
I understand your motivation but I don'
On Mon, May 20, 2013 at 5:55 PM, Carlos Nepomuceno
wrote:
> I understand your motivation but I don't know what protection
> ast.literal_eval() is offering that eval() doesn't.
eval will *execute code*, while literal_eval will not. That's the
protection. With ast.literal_eval, all that can happen
On 20/05/2013 09:55, Chris Angelico wrote:
On Mon, May 20, 2013 at 5:50 PM, Frank Millman wrote:
On 20/05/2013 09:34, Carlos Nepomuceno wrote:
Why don't you use eval()?
Because users can create their own columns, with their own constraints.
Therefore the string is user-modifiable, so it can
> To: python-list@python.org
> From: fr...@chagford.com
> Subject: Re: Question about ast.literal_eval
> Date: Mon, 20 May 2013 09:50:02 +0200
>
> [Corrected top-posting]
>
>>> To: python-list@python.org
>>> From: f
On Mon, May 20, 2013 at 5:50 PM, Frank Millman wrote:
> On 20/05/2013 09:34, Carlos Nepomuceno wrote:
>> Why don't you use eval()?
>>
>
> Because users can create their own columns, with their own constraints.
> Therefore the string is user-modifiable, so it cannot be trusted.
Plenty of reason ri
[Corrected top-posting]
>> To: python-list@python.org
From: fr...@chagford.com
Subject: Question about ast.literal_eval
Date: Mon, 20 May 2013 09:05:48 +0200
Hi all
I am trying to emulate a SQL check constraint in Python. Quoting from
the PostgreSQL docs, "A check constraint is the most generi
On Mon, May 20, 2013 at 5:05 PM, Frank Millman wrote:
> Hi all
>
> I am trying to emulate a SQL check constraint in Python. Quoting from the
> PostgreSQL docs, "A check constraint is the most generic constraint type. It
> allows you to specify that the value in a certain column must satisfy a
> Bo
It seems to me you can't use ast.literal_eval()[1] to evaluate that kind of
expression because it's just for literals[2].
Why don't you use eval()?
[1] http://docs.python.org/2/library/ast.html#ast-helpers
[2] http://docs.python.org/2/reference/lexical_analysis.html#literals
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