How can I check to see if a file exists before proceeding using Python

Question

I have some code that will find the newest file in a directory and append a time stamp to the file name. It works great as long as there is a file in the directory to rename. If there isn't I am getting:

"ValueError: max() arg is an empty sequence"

Here's my code:

import os
import glob
import datetime
now = datetime.datetime.now()
append = now.strftime("%H%M%S")
newest = max(glob.iglob('1234_fileName*.LOG'), key=os.path.getmtime)
newfile = (append+"_"+newest)
os.rename(newest, newfile)

Any suggestions for simplifying the code would be appreciated as well as explaining how to only run if a "1234_fileName*.LOG" (note the wildcard) file is detected.

What I need this program to do is run periodically (I can use task scheduler for that) and check for a new file. If there is a new file append the hours, minutes and seconds to it's name.

Thanks!

could you edit your title to reflect the question text? If you want to preserve the title then it is a duplicate of: [Check if a file exists using Python](http://stackoverflow.com/q/82831/4279) — jfs, Dec 23 '14 at 00:18
related: [Check the permissions of a file in python](http://stackoverflow.com/q/27434643/4279). — jfs, Dec 23 '14 at 00:23

score 3 · Accepted Answer · answered Dec 23 '14 at 00:12

3

You could use glob.glob() that returns a list instead of glob.iglob() that returns an iterator:

files = glob.glob('1234_fileName*.LOG')
if files:
   newest = max(files, key=os.path.getmtime)
   newfile = append + "_" + newest
   os.rename(newest, newfile)

Both glob() and iglob() use os.listdir() under the hood so there is no difference for a single directory.

answered Dec 23 '14 at 00:12

jfs

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Thank you. Works exactly like I want now. – urtonbay Dec 23 '14 at 00:35

Lee Daniel Crocker · Answer 2 · 2014-12-23T00:22:57.837

2

max() is complaining that you're asking for the largest of 0 items, and throwing a ValueError. You'll have to catch it. This will continue to throw any IOErrors that might occur:

import os, glob, datetime

try:
    app = datetime.datetime.now().strftime("%H%M%S")
    newest = max(glob.iglob("1234_filename*.LOG"), key=os.path.getmtime)
    newfile = (app + "_" + newest)
    os.rename(newest, newfile)
except ValueError:
    pass

edited Dec 23 '14 at 00:22

answered Dec 23 '14 at 00:10

Lee Daniel Crocker

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`files` is always true in your example because `iglob()` returns an iterator – jfs Dec 23 '14 at 00:13
You're right, iterators can't be tested that way. OK, then, back to try/except. – Lee Daniel Crocker Dec 23 '14 at 00:23
Thank you. This helped me clean up my code. However, I still was getting the same error. Changing files = glob.iglob to files = glob.glob worked. – urtonbay Dec 23 '14 at 00:38

score 1 · Answer 3 · answered Dec 23 '14 at 00:09

1

os.access allows you to check access rights before operations. An example is right under the link.

Also, it's fine to just do things inside a try .. except IOError.

answered Dec 23 '14 at 00:09

9000

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Plus one for `try`-`catch`, that's allegedly more Pythonic and it seems as if it would be [more resistant to race conditions](http://blog.rodneyrehm.de/archives/12-Improving-Disk-IO-in-PHP-Apps.html). – Waleed Khan Dec 23 '14 at 00:11
how `os.access()` may help to avoid `"ValueError: max() arg is an empty sequence"` in this case? – jfs Dec 23 '14 at 00:18
@J.F.Sebastian: Sorry, I was answering the title question, "How can I check to see if a file exists before proceeding using Python", not the implied question "why this code fails". – 9000 Dec 23 '14 at 16:18

How can I check to see if a file exists before proceeding using Python

3 Answers3