Check that a *type* of file exists in Python -

i realize looks similar other questions checking if file exists, different. i'm trying figure out how check type of file exists , exit if doesn't. code tried this:

filenames = os.listdir(os.curdir)  filename in filenames:     if os.path.isfile(filename) , filename.endswith('.fna'):         ##do stuff 

this works 'do stuff' file ends in .fna, need check , make sure .fna file there , exit program entirely if not.

i tried this:

try:      if os.path.isfile(filename) , filename.endswith('.fna'):         ## stuff  except:       sys.stderr.write (‘no database file found. exiting program. /n’)     sys.exit(-1) 

but didn't work, skips whole function if .fna file isn't there, without printing error.

the for statement in python has little-known else clause:

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

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