variables exist

[Steven Bethard]

Brian van den Broek wrote:
> Fredrik Lundh said unto the world upon 2005-04-11 10:14:
>> "fabian" wrote:
>>> how testing if a variable exists in python as isset in php??
>>
>> try:
>>     variable
>> except NameError:
>>     print "variable not set"
>>
>> but that is really lousy Python; better make sure you always assign to
>> all variables, and use None (or another suitable value) to mark that some
>> variable has no meaningful content right now.
>>
>> that is, instead of
> <SNIP full try/except example>
>> write
>>
>>     variable = None
>>     if condition:
>>         variable = 1
>>     ...
>>     if variable is None:
>>         print "variable not set"
> 
> I'm a hobbyist and still learning, but the claim the try/except is
> "lousy Python" surprise me a bit. The try/except way seems like an
> instance of "Easier to ask Forgiveness than Permission", and my sense
> is that in 95% of cases that approach is widely considered more Pythonic 
> than LBYL (of which the suggested idiom seems reminiscent).
> 
> As it happens, most of my code does preset names to None, but reading
> this thread made me wonder. Are you suggesting setting to None is
> better as it makes explicit that there is nothing there, but this may
> change in the future? (This is why I do it.) Or for some other reason?

I had a similar reaction to Fredrik's statement.  In general, I wouldn't 
call a try/except solution lousy style, as long as the path through 
except was sufficiently "exceptional".  That is, for code like:

     if condition_that_happens_99_percent_of_the_time:
         variable = 1
     try:
         variable
     except NameError:
         print "variable not set"

seems perfectly reasonable to me, because the exception is caught in the 
*exceptional* case.  However, if one case isn't clearly the exceptional 
one, then it probably doesn't makes sense to use an exception, and I'd 
write it like:

     variable = None
     if condition_that_happens_40_percent_of_the_time:
         variable = 1
     ...
     if variable is None:
         print "variable not set"

Actually, I typically refactor my code so that I can write situations 
like this as:

     if condition_that_happens_40_percent_of_the_time:
         variable = 1
         ...
     else:
         print "variable not set"

But the point still stands.  If the variable being unset really is an 
exceptional case, use an exception.  If it's not, then an if/else block 
is probably the better way to go.

STeVe