Check if a directory contains a file with a given extension

Question

I need to check the current directory and see if a file with an extension exists. My setup will (usually) only have one file with this extension. I need to check if that file exists, and if it does, run a command.

However, it runs the else multiple times because there are multiple files with alternate extensions. It must only run the else if the file does not exist, not once for every other file. My code sample is below.

---

The directory is structured like so:

dir_________________________________________
    \            \            \            \     
 file.false    file.false    file.true    file.false

When I run:

import os
for File in os.listdir("."):
    if File.endswith(".true"):
        print("true")
    else:
        print("false")

The output is:

false
false
true
false

The issue with this is if I replaced print("false") with something useful, it will run it multiple times.

Edit: I asked this question 2 years ago, and it's still seeing very mild activity, therefore, I'd like to leave this here for other people: http://book.pythontips.com/en/latest/for_-_else.html#else-clause

Answer 1

You can use the else block of the for:

for fname in os.listdir('.'):
    if fname.endswith('.true'):
        # do stuff on the file
        break
else:
    # do stuff if a file .true doesn't exist.

The else attached to a for will be run whenever the break inside the loop is not executed. If you think a for loop as a way to search something, then break tells whether you have found that something. The else is run when you didn't found what you were searching for.

Alternatively:

if not any(fname.endswith('.true') for fname in os.listdir('.')):
    # do stuff if a file .true doesn't exist

---

Moreover you could use the glob module instead of listdir:

import glob
# stuff
if not glob.glob('*.true')`:
    # do stuff if no file ending in .true exists