Python file cache

Question

I\'m creating some objects from files (validators from templates xsd files, to draw together other xsd files, as it happens), and I\'d like to recreate the objects when the

Answer 1

Three thoughts.

1. Use try... except... else for a neater control flow.

2. File modification times are notoriously unstable -- in particular, they don't necessarily correspond to the most recent time the file was modified!

3. Python 3 contains a caching decorator: functools.lru_cache. Here's the source.

   def lru_cache(maxsize=100):
       """Least-recently-used cache decorator.
   
       If *maxsize* is set to None, the LRU features are disabled and the cache
       can grow without bound.
   
       Arguments to the cached function must be hashable.
   
       View the cache statistics named tuple (hits, misses, maxsize, currsize) with
       f.cache_info().  Clear the cache and statistics with f.cache_clear().
       Access the underlying function with f.__wrapped__.
   
       See:  http://en.wikipedia.org/wiki/Cache_algorithms#Least_Recently_Used
   
       """
       # Users should only access the lru_cache through its public API:
       #       cache_info, cache_clear, and f.__wrapped__
       # The internals of the lru_cache are encapsulated for thread safety and
       # to allow the implementation to change (including a possible C version).
   
       def decorating_function(user_function,
                   tuple=tuple, sorted=sorted, len=len, KeyError=KeyError):
   
           hits = misses = 0
           kwd_mark = (object(),)          # separates positional and keyword args
           lock = Lock()                   # needed because ordereddicts aren't threadsafe
   
           if maxsize is None:
               cache = dict()              # simple cache without ordering or size limit
   
               @wraps(user_function)
               def wrapper(*args, **kwds):
                   nonlocal hits, misses
                   key = args
                   if kwds:
                       key += kwd_mark + tuple(sorted(kwds.items()))
                   try:
                       result = cache[key]
                       hits += 1
                   except KeyError:
                       result = user_function(*args, **kwds)
                       cache[key] = result
                       misses += 1
                   return result
           else:
               cache = OrderedDict()       # ordered least recent to most recent
               cache_popitem = cache.popitem
               cache_renew = cache.move_to_end
   
               @wraps(user_function)
               def wrapper(*args, **kwds):
                   nonlocal hits, misses
                   key = args
                   if kwds:
                       key += kwd_mark + tuple(sorted(kwds.items()))
                   try:
                       with lock:
                           result = cache[key]
                           cache_renew(key)        # record recent use of this key
                           hits += 1
                   except KeyError:
                       result = user_function(*args, **kwds)
                       with lock:
                           cache[key] = result     # record recent use of this key
                           misses += 1
                           if len(cache) > maxsize:
                               cache_popitem(0)    # purge least recently used cache entry
                   return result
   
           def cache_info():
               """Report cache statistics"""
               with lock:
                   return _CacheInfo(hits, misses, maxsize, len(cache))
   
           def cache_clear():
               """Clear the cache and cache statistics"""
               nonlocal hits, misses
               with lock:
                   cache.clear()
                   hits = misses = 0
   
           wrapper.cache_info = cache_info
           wrapper.cache_clear = cache_clear
           return wrapper
   
       return decorating_function