on_modified_decorator.py

import os
import time
from functools import wraps

def memoize_until_file_modified(filepath):
    """"Memoize the data until the mtime of the file at filepath changes."""
    def memoize_until_file_modified_decorator(func):
        @wraps(func)
        def decorated(*args, **kwargs):
            try:
                mtime = int(os.stat(filepath).st_mtime)
                if decorated._last_mtime[filepath] != mtime:
                    decorated._last_mtime[filepath] = mtime
                    decorated._last_result = func(*args, **kwargs)
            except AttributeError:
                decorated._last_mtime = {}
                decorated._last_mtime[filepath] = mtime
                decorated._last_result = func(*args, **kwargs)
            return decorated._last_result
        return decorated
    return memoize_until_file_modified_decorator


# Example Usage

@memoize_until_file_modified('test.txt')
def get_first_char_in_data():
    print 'reading "%s" contents' % 'test'
    data = file('test.txt').read()
    return data[0]

@memoize_until_file_modified('test2.txt')
def get_second_last_char_in_data():
    print 'reading "%s" contents' % 'test2'
    data = file('test2.txt').read()
    print 'read data :%s' % data
    return data[-2]

file('test.txt', 'w').write('original content')
file('test2.txt', 'w').write('original content')

while True:
    print '-'*60
    print get_first_char_in_data()
    print get_second_last_char_in_data()
    print '-'*60
    time.sleep(3)