Check that a *type* of file exists in Python

Question

I realize this looks similar to other questions about checking if a file exists, but it is different. I'm trying to figure out how to check that a type of file exists and exit if it doesn't. The code I tried originally is this:

filenames = os.listdir(os.curdir)

for filename in filenames:

   if os.path.isfile(filename) and filename.endswith('.fna'):
        ##do stuff

This works to 'do stuff' to the file that ends in .fna, but I need it to check and make sure the .fna file is there and exit the program entirely if not.

I tried this:

try:

    if os.path.isfile(filename) and filename.endswith('.fna'):
        ## Do stuff 
except: 

    sys.stderr.write (‘No database file found. Exiting program. /n’)
    sys.exit(-1)

But that didn't work, it just skips the whole function if the .fna file isn't there, without printing the error.

Answer 1

The for statement in Python has a little-known else clause:

for filename in filenames:
    if os.path.isfile(filename) and filename.endswith(".fna"):
        # do stuff
        break
else:
    sys.stderr.write ('No database file found. Exiting program. \n')
    sys.exit(-1)

The else clause is run only if the for statement runs through its whole enumeration without executing the break inside the loop.