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

Answer 1

Look at the glob module:

import glob
import os
import sys

databases = filter(os.path.isfile, glob.glob('./*.fna'))

if not databases:
    sys.stderr.write("No database found.\n\n")
    exit(1)

for database in databases:
    do_something_with(database)

Answer 2

filenames = os.listdir(os.curdir)
found = False
for filename in filenames:
    if os.path.isfile(filename) and filename.endswith('.fna'):
        found = True
if not found:
    sys.stderr.write ('No database file found. Exiting program. \n')
    sys.exit(-1)

Answer 3

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.

Answer 4

If you are using exceptions, do not test to see if the file exists. That's why you're using exceptions to start with.

try:
    # No if statement doing the check
    # Start doing stuff assuming abc.fna exists
except:
    # Uh oh! Something went bad.
    # Display error messages, clean up, carry on

To clarify, consider the code snippet:

try:
    with open('hi.txt') as f:
        print f.readlines()
except:
    print 'There is no hi.txt!'

Answer 5

Your try/except block didn't work because os.path.isfile does not throw an exception if it fails; it merely returns False.

else clauses for for loops are weird and non-intuitive. Using break to signify that the loop was successful rather than legitimately breaking it is just weird, and contrary to Python's philosophy.

Here's a nice, Pythonic way of doing what you want:

want = lambda f: os.path.isfile(f) and f.endswith(".fna")
valid_files = [f for f in os.listdir(os.curdir) if want(f)]
if len(valid_files) == 0:
    print >>sys.stderr, "failed to find .fna files!"
    sys.exit(1)
for filename in valid_files:
    # do stuff

Answer 6

Check out os.path.splitext(path) function which says:

Split the pathname path into a pair (root, ext) such that root + ext == path, and ext is empty or begins with a period and contains at most one period. Leading periods on the basename are ignored; splitext('.cshrc') returns ('.cshrc', '').

Here's an example:

>>> os.path.splitext("./01The News from Lake Wobegon/AlbumArtSmall.jpg")
('./01The News from Lake Wobegon/AlbumArtSmall', '.jpg')
>>>