Python: Apply if statement from string

Question

I have an if statement in a .ini file:

critical= $TREND$>=40

In my script I replace $TREND$ by my value so I have in a variable:

critical = "10.5>=40"

I try to execute this if statement to know if it's True or False.

if critical:
   print "It's critical!"

(I know it will just check if critical is empty)

You could eval the string, but that's potentially risky - the .ini file could contain anything. Why not parse it? — jonrsharpe
– jonrsharpe, Commented Mar 11, 2015 at 16:19
i would advice you to have critical_low a critical_high in your .ini which will make your code better. eval is evil — user3012759
– user3012759, Commented Mar 11, 2015 at 16:24

lsowen · Accepted Answer · 2015-03-11 16:22:42Z

2

One option is to use eval:

critical = eval("10.5>=40")

However, you must be sure that what is "eval'ed" is completely trusted, because it allows you to execute any arbitrary python code. That is, don't use this if any part of the string is passed in by an "untrusted entity" (user, external api, etc).

Another, possibly better, option might be to use a "template parser" which allows "untrusted" or "sandboxed" execution. For example, the Jinja Sandbox mode.

answered Mar 11, 2015 at 16:22

lsowen

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1 Comment

B3luT Over a year ago

Indeed I know this is bad in a security perspective but this is awesome! Thanks.

user3012759 · Accepted Answer · 2015-03-11 16:28:34Z

0

I would urge you to change your .ini to have following:

critical_low

critical_high

this way you avoid two problems - you don't have to use eval which can be very dangerous, and you don't have logic in your .ini unless you do want logic there which still can be solved without using eval

said that eval will work fine if it's just a hacky script

answered Mar 11, 2015 at 16:28

user3012759

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2 Comments

B3luT Over a year ago

It's just I need the operator. Sometime critical will be $VALUE$<1

user3012759 Over a year ago

fair enough :) you can always add critical_operator but it will bloat your code somehow. guess i just really don't like eval :)

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