boB Stepp wrote:
> 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()?
No. To expand on Marks hint here's how to do it without evil eval().
import operator
comps = {
"=": operator.eq,
"<": operator.lt,
">": operator.gt,
# ...
}
def choose_compare(operator, value0, value1, pass_color, fail_color):
op = comps[operator]
if op(value0, value1):
return pass_color, True
else:
return fail_color, False
print(choose_compare("=", 1, 1, "red", "blue"))
print(choose_compare("<", 1, 2, "red", "blue"))
print(choose_compare("<", 2, 1, "red", "blue"))
Rule of thumb: when you think you need eval() you're wrong.
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