Python function to match filenames with extension names

Question

I have written a Python function which matches all files in the current directory with a list of extension names. It is working correctly.

import os, sys, time, re, stat

def matchextname(extnames, filename):
    # Need to build the regular expression from the list
    myregstring = ""
    for index in range(len(extnames)):
        # r1 union r2 and  so on operator is pipe(|)
        # $ is to match from the end
        if index <  len(extnames) - 1:
            myregstring =  myregstring + extnames[index] + '$' + '|'
        else:
            myregstring = myregstring + extnames[index] + '$'
    # getting regexobject
    myregexobj = re.compile(myregstring)
    # Now search
    searchstat = myregexobj.search(filename)
    if searchstat:
        print 'Regex', filename

It is called like this:

if __name__ == '__main__':
    fileextensions = ['\.doc', '\.o', '\.out', '\.c', '\.h']
    try:
        currentdir = os.getcwd()
    except OSError:
        print 'Error occured while getting current directory'
        sys.exit(1)
    for myfiles in os.listdir(currentdir):
        matchextname(fileextensions, myfiles)

Could you please review the code and suggest if there is any better way of doing this, or share any other comments related to errors/exception handling which are missing - or anything else in terms of logic?

Gareth Rees · Accepted Answer · 2012-11-28 15:14:21Z

I think the way to code this, that most clearly indicates what you are doing, is to use os.path.splitext to get the extension, and then look it up in a set of extensions:

import os.path
extensions = set('.doc .o .out .c .h'.split())
_, ext = os.path.splitext(filename)
if ext in extensions:
    print(filename)

A couple of other comments on your code:

There's no need to catch the OSError if all you're going to do is print a message and exit. (This will happen in any case if the exception is uncaught, so why go to the extra trouble?)
If you actually want to build up a regular expression, then do it using str.join. This avoids the need to have a special case at the end:
```
extensions = r'\.doc \.o \.out \.c \.h'.split()
myregexobj = re.compile('(?:{})$'.format('|'.join(extensions)))
```
(Whenever you find yourself writing a loop over the indexes to a sequence, then you should think about rewriting it to loop over the elements of the sequence instead: this nearly always results in clearer and shorter code.)
If you want to build a regular expression that exactly matches a string literal, you should use re.escape to escape the special characters, instead of escaping each one by hand. For example:
```
extensions = '.doc .o .out .c .h'.split()
myregexobj = re.compile('(?:{})$'.format('|'.join(map(re.escape, extensions))))
```

Thanks Gareth. Your explanation and all the methods that you have shared are excellent for learning. These really helped me to understand many new and better techniques. Thanks again. — Tanmoy Bandyopadhyay
– Tanmoy Bandyopadhyay, Commented Nov 28, 2012 at 18:18

jfs · Accepted Answer · 2012-11-28 11:29:36Z

7

.endswith() accepts a tuple:

#!usr/bin/env python
import os

fileextensions = ('.doc', '.o', '.out', '.c', '.h')
for filename in os.listdir(os.curdir):
    if filename.endswith(fileextensions):
       print(filename)

answered Nov 28, 2012 at 11:29

jfs

7333 silver badges12 bronze badges

\$\begingroup\$ Sebastian. Thanks a lot for your help. This is quite a nice approach, which you have shown \$\endgroup\$

Tanmoy Bandyopadhyay
– Tanmoy Bandyopadhyay

2012-11-28 18:15:32 +00:00
Commented Nov 28, 2012 at 18:15

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