Speed ain't bad

[Jeff Shannon]

Anders J. Munch wrote:

> Another way is the strategy of "it's easier to ask forgiveness than to
> ask permission".
> If you replace:
>     if(not os.path.isdir(zfdir)):
>         os.makedirs(zfdir)
> with:
>     try:
>         os.makedirs(zfdir)
>     except EnvironmentError:
>         pass
> 
> then not only will your script become a micron more robust, but
> assuming zfdir typically does not exist, you will have saved the call
> to os.path.isdir.

... at the cost of an exception frame setup and an incomplete call to 
os.makedirs().  It's an open question whether the exception setup and 
recovery take less time than the call to isdir(), though I'd expect 
probably not.  The exception route definitely makes more sense if the 
makedirs() call is likely to succeed; if it's likely to fail, then 
things are murkier.

Since isdir() *is* a disk i/o operation, then in this case the 
exception route is probably preferable anyhow.  In either case, one 
must touch the disk; in the exception case, there will only ever be 
one disk access (which either succeeds or fails), while in the other 
case, there may be two disk accesses.  However, if it wasn't for the 
extra disk i/o operation, then the 'if ...' might be slightly faster, 
even though the exception-based route is more Pythonic.

Jeff Shannon
Technician/Programmer
Credit International