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

Question

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"
else:
    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.

Answer 1

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

Answer 2

I believe that throwing an exception is a better idea for your situation. An alternative will be the simulation method to return a tuple. The first item will be the status and the second one the result:

result = simulate(open("myfile"))
if not result[0]:
  print "error parsing stream"
else:
  ret= result[1]

Answer 3

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

try:
    result = simulate(open("myfile"))
except SimulationException:
    print "error parsing stream"
else:
    if result:
        print "result pass"
    else:
        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.

Answer 4

Never, never, never say

if something == True:

Never. It's crazy, since you're redundantly repeating what is redundantly specified as the redundant condition rule for an if-statement.

Worse, still, never, never, never say

if something == False:

You have not. Feel free to use it.

Finally, doing a == None is inefficient. Do a is None. None is a special singleton object, there can only be one. Just check to see if you have that object.