[Tutor] How to use try and except in this case?

[Steven D'Aprano]

Mic wrote:

> Say that I want to try and open 10 files. If none of these exists, I 
> want an error
> message to appear. But only if NONE of these files exists.
> 
> I know how to handle this with one file. But I don't know how to do that 
> with more than one.
> So the program should try and open all 10 files and if, and only if, 
> none of the files exists I want en error message to appear.

Your description isn't quite clear enough. What happens if, say, only 3 of the 
files exist? Is the intention to open the first file that you can find?

# Ten file names to try:
names = ['file 1', 'file 2', 'a', 'b', 'c', 'backup file',
          'backup file 2', 'spam', 'ham', 'eggs'
          ]
# Find the first one that exists and is readable.
for name in names:
     try:
         f = open(name, 'r')
     except (OSError, IOError):
         continue  # try the next one
     break  # this only runs when a file opens successfully
else:
     # we didn't break
     raise ValueError('no file could be opened')



Or is it your intention to open as many of the files as possible?


# Ten file names to try:
names = ['file 1', 'file 2', 'a', 'b', 'c', 'backup file',
          'backup file 2', 'spam', 'ham', 'eggs'
          ]
files = []  # list of opened file objects
for name in names:
     try:
         f = open(name, 'r')
         files.append(f)
     except (OSError, IOError):
         continue  # missing, go on to the next one
if files == []:
     raise ValueError('no file could be opened')
else:
     # do stuff with the files...
     # ...
     # ...
# don't forget to close them when done (optional but recommended)
for f in files:
     f.close()




-- 
Steven