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

User · Answer

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

User · Answer

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

User · Answer

Don t fear the Exception   Having your program just log and continue is as easy as   try      result   simulate open  myfile    except SimulationException as sim exc      print  error parsing stream   sim exc 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

User · Answer

There are many good answers  I would like to add one more point  A bug can get into your code if you are working with numerical values  and your answer is happened to be 0  a   0  b   10  c   None      Common approach that can cause a problem  if not a      print f quot Answer is not found  Answer is  str a    quot    else      print f quot Answer is   str a    quot    if not b      print f quot Answer is not found  Answer is  str b    quot    else      print f quot Answer is   str b   quot    if not c      print f quot Answer is not found  Answer is  str c    quot    else      print f quot Answer is   str c    quot    Answer is not found  Answer is 0     Answer is  10     Answer is not found  Answer is None       Safer approach  if a is None      print f quot Answer is not found  Answer is  str a    quot    else      print f quot Answer is   str a    quot    if b is None      print f quot Answer is not found  Answer is  str b    quot    else      print f quot Answer is   str b    quot    if c is None      print f quot Answer is not found  Answer is  str c    quot    else      print f quot Answer is   str c    quot     Answer is  0  Answer is  10  Answer is not found  Answer is None

User · Answer

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

User · Answer

I would like to stress that  even if there are situations where if expr   isn t sufficient because one wants to make sure expr is True and not just different from 0 None whatever  is is to be prefered from    for the same reason S Lott mentionned for avoiding    None   It is indeed slightly more efficient and  cherry on the cake  more human readable   In  1    timeit  1    1     True 38 1 ns    0 116 ns per loop  mean    std  dev  of 7 runs  10000000 loops each   In  2    timeit  1    1  is True 33 7 ns    0 141 ns per loop  mean    std  dev  of 7 runs  10000000 loops each

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

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