import errno
import gzip
import md5
import os

import Conf

class CacheMissException (Exception):
  pass
 
class CacheObject:
  """ A CacheObject is a compressed on-disk version of a serialized object.
  """
  def __init__ (self, key=None, keys=None):
    self.obj = None
    #print "CacheObject: key=%s keys=%s"%(key, keys)
    if not key and not keys:
      raise "CacheObject needs at least one of key, keys"
    checksum = md5.md5()
    if key:
      checksum.update(key)
    if keys:
      for k in keys:
        checksum.update(str(k))
    self.path = os.path.sep.join([Conf.cachedir, checksum.hexdigest()])

  def get(self):
    if not self.obj:
      try:
        if Conf.cachecompress:
          self.obj = gzip.GzipFile(self.path).read()
        else:
          self.obj = file(self.path).read()
      except IOError, e:
        if e.errno != errno.ENOENT:
          raise e
        else:
          #print "CacheObject: cache miss"
          raise CacheMissException("Object not present in cache")
    #print "CacheObject: cache hit"
    return self.obj

  def put(self, obj):
    self.obj = obj
    #print "CacheObject: cache put"
    try:
      if Conf.cachecompress:
        gzip.GzipFile(self.path, "wb").write(str(self.obj))
      else:
        file(self.path, "wb").write(str(self.obj))
    except Exception, e:
      os.unlink(self.path)
      raise e

if __name__ == '__main__':
  co = CacheObject( key="magic" )
  try:
    print "going to try to read an object before it exists"
    print "first line:", co.get().split()[0]
  except CacheMissException:
    print "file was missing as expected.  now let's try to put it and fetch it"
    co.put(file("/etc/passwd").read())
    print "first line:", co.get().split()[0]