A large Python application includes resource files (e.g., Glade project files, SQL templates, and images) as well as Python packages. You want to store these associated files together with the Python packages that use them.

You need to be able to look for either files or directories along Python’s sys.path:

import sys, os

class Error(Exception): pass

def _find(pathname, matchFunc=os.path.isfile):
    for dirname in sys.path:
        candidate = os.path.join(dirname, pathname)
        if matchFunc(candidate):
            return candidate
    raise Error("Can't find file %s" % pathname)

def findFile(pathname):
    return _find(pathname)

def findDir(path):
    return _find(path, matchFunc=os.path.isdir)

Larger Python applications consist of sets of Python packages and associated sets of resource files. It’s convenient to store these associated files together with the Python packages that use them, and it’s easy to do so if you use this variation on Recipe 4.21 to find files or directories with pathnames relative to the Python search path.

Recipe 4.21; documentation for the os module in the Library Reference.