Read directory listing of files and parse out the highest number?

[Larz60+]

Another approach:
test conditions:
directory in same path as script, named 'filedir' (you can change as needed)
containing the following files:

Output:
igMess_9.csv
igMess_10.csv
igMess_11_.csv
igMess_12_.csv
igMess_13_.csv

from pathlib import Path
import os


os.chdir(os.path.abspath(os.path.dirname(__file__)))

def check_for_highest(StartsWith, EndsWith, Directory):
    dir = Path(Directory)
    filelist = [filename for filename in dir.iterdir() \
        if filename.is_file() and str(filename.stem).startswith(StartsWith) \
        and str(filename.stem).endswith(EndsWith)]
    largest_name = sorted(filelist)[-1]
    return filelist, largest_name

def testit():
    filelist, largest = check_for_highest('igMess_', '_', 'filedir')

    for file in filelist:
        print(file)

    next_available = str(int(largest.stem[7:-1]) + 1)
    print(f"next available number = {next_available}")

testit()

results:

Output:
filedir/igMess_11_.csv
filedir/igMess_12_.csv
filedir/igMess_13_.csv
next available number = 14