Find a value between two of the same characters in a string using Python
Rather than using str.find, you could better describe yourself using regex. However it's not that much of an improvement.

For example using the regex _(.+)_ on the basename of the file is all you need. If you think a file extension is going to have an _ then you may need the splitext.

This can get:

from os.path import splitext, basename
from re import search

base = basename(splitext(filename_goes_here)[0])
value = search('_(.+)_', base)
if value is not None:
    value = value.group(1)


If you're using Python 3.6, as noted in the comments by 200_success, you could change the last line to:

value = value[0]