On Wed, Apr 29, 2015 at 3:46 PM, Marc Tompkins <marc.tompk...@gmail.com> wrote:
> On Wed, Apr 29, 2015 at 1:10 PM, boB Stepp <robertvst...@gmail.com> wrote:
>>
>> Python 2.4.4, Solaris 10.
>>
>> I have some functions that I believe I could collapse into a single
>> function if I only knew how:
>>
>> def choose_compare(operator, value0, value1, pass_color, fail_color):
>> """
>> Perform the comparison indicated by operator. Return pass_color if
>> true, fail_color if false.
>> """
>> if operator == '<':
>> return less_than(value0, value1, pass_color, fail_color)
>> elif operator == '<=':
>> return less_than_or_equal(value0, value1, pass_color, fail_color)
>> elif operator == '=':
>> return equal(value0, value1, pass_color, fail_color)
>> elif operator == '>':
>> return greater_than(value0, value1, pass_color, fail_color)
>> elif operator == '>=':
>> return greater_than_or_equal(value0, value1, pass_color,
>> fail_color)
>> else:
>> print 'WarningMessage = "Invalid comparison operator in
>> function, choose_compare().@Please contact script administrator for
>> assistance.";'
>>
>> def less_than(value0, value1, pass_color, fail_color):
>> """
>> See if value0 is less than value1. If true, return pass_color. If
>> false, return fail_color.
>> """
>> if value0 < value1:
>> return pass_color, True
>> else:
>> return fail_color, False
>>
>> def less_than_or_equal(value0, value1, pass_color, fail_color):
>> """
>> See if value0 is less than or equal to value1. If true, return
>> pass_color. If false, return fail_color.
>> """
>> if value0 <= value1:
>> return pass_color, True
>> else:
>> return fail_color, False
>>
>> ... 3 more functions ...
>>
>> I won't give the remaining functions for the other comparison
>> operators. The string variable, operator, is originally populated from
>> a data file, which tells what type of comparison needs to be made. The
>> last two functions I gave (and the ones I omitted giving) all follow
>> the same exact pattern. I know there has to be some way to replace
>> these 5 functions with 1, but what experimentation I have done to date
>> has not worked.
>>
>> Also, what about the first function above? I could use 2 dictionaries,
>> 1 for calling the 5 functions and one to pass the arguments, but is it
>> worth doing this? Or, I would not be surprised if there is a much
>> better way! ~(:>))
>>
>> Thanks!
>
>
> Here's what I came up with:
>
> def choose_compare(operator, value0, value1, pass_color, fail_color):
> comps = {"=":"==", "<":"<", ">":">", "<=":"<=", ">=":">="}
> if operator in comps.keys():
> operator = comps[operator]
> if eval("{} {} {}".format(value0, operator, value1)):
> return pass_color, True
> else:
> return fail_color, False
> else:
> print('WarningMessage')
>
> I would ordinarily avoid eval() like the plague, but I think that this
> sanitizes the input pretty effectively. I had to make comps a dict instead
> of a list because (in your example, anyway) you're using a single equals
> sign to check for equality, which throws a Syntax Error (e.g. "if 1 = 2"
> instead of "if 1 == 2").
I could deal with the "=" issue by either reformatting my data file to
use "==" in place of "=", or when I parse the data file, do the
replacement there. A list instead of the dictionary looks a little
easier on my eyes.
The list has me so leery of eval and exec that I totally forgot about
this possibility! There are only two places in my program where I read
information directly into my program: 1) The data file, or 2) how the
user of the planning software names his regions of interest (ROI) in
the planning system software. I will reexamine my checks of (1). For
(2) the planning software already has its own checks, which would
filter out a lot. And I am checking the ROIs to see if they are
present in the data file *exactly* as given in the data file;
otherwise, I reject them.
So I have stumbled (With your gracious help!) into a legitimate use of eval()?
Many thanks, again!
--
boB
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