In Python how should I test if a variable is None, True or False


I have a function that can return one of three things:

  • success (True)
  • failure (False)
  • error reading/parsing stream (None)

My question is, if I'm not supposed to test against True or False, how should I see what the result is. Below is how I'm currently doing it:

result = simulate(open("myfile"))
if result == None:
    print "error parsing stream"
elif result == True: # shouldn't do this
    print "result pass"
    print "result fail"

is it really as simple as removing the == True part or should I add a tri-bool data-type. I do not want the simulate function to throw an exception as all I want the outer program to do with an error is log it and continue.

1/7/2010 1:32:36 PM

Accepted Answer

Don't fear the Exception! Having your program just log and continue is as easy as:

    result = simulate(open("myfile"))
except SimulationException as sim_exc:
    print "error parsing stream", sim_exc
    if result:
        print "result pass"
        print "result fail"

# execution continues from here, regardless of exception or not

And now you can have a much richer type of notification from the simulate method as to what exactly went wrong, in case you find error/no-error not to be informative enough.

10/14/2015 8:13:55 PM

if result is None:
    print "error parsing stream"
elif result:
    print "result pass"
    print "result fail"

keep it simple and explicit. You can of course pre-define a dictionary.

messages = {None: 'error', True: 'pass', False: 'fail'}
print messages[result]

If you plan on modifying your simulate function to include more return codes, maintaining this code might become a bit of an issue.

The simulate might also raise an exception on the parsing error, in which case you'd either would catch it here or let it propagate a level up and the printing bit would be reduced to a one-line if-else statement.

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