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from logging import getLogger

log = getLogger("qpid.saslmech")

def get_sasl_mechanism(mechanismNames, username, password, namespace="qpid.saslmech", sasl_options=None):
  """Given a list of SASL mechanism names, dynamically loads a SASL implementation
     from namespace qpid.sasl.mech respecting a mechanism priority"""

  log.debug("Supported mechanism : %s", mechanismNames)

  instances = []
  for mechanismName in mechanismNames:
    convertedName = mechanismName.replace("-","_")
    canonicalName = "%s.%s.%s" % (namespace, convertedName.lower(), convertedName)
    try:
      log.debug("Checking for SASL implementation %s for mechanism %s", canonicalName, mechanismName)
      clazz = _get_class(canonicalName)
      log.debug("Found SASL implementation")
      instance = clazz(username, password, mechanismName, sasl_options)
      if (instance.prerequisitesOk()):
        instances.append(instance)
      else:
        log.debug("SASL mechanism %s unavailable as the prerequistes for this mechanism have not been met", mechanismName)
    except (ImportError, AttributeError), e:
      # Unknown mechanism - this is normal if the server supports mechanism that the client does not
      log.debug("Could not load implementation for %s", canonicalName)
      pass

  if instances:
    instances.sort(key=lambda x : x.priority(), reverse=True)
    sasl = instances[0]
    log.debug("Selected SASL mechanism %s", sasl.mechanismName())
    return sasl
  else:
    return None

def _get_class( kls ):
  parts = kls.split('.')
  module = ".".join(parts[:-1])
  m = __import__( module )
  for comp in parts[1:]:
    m = getattr(m, comp)
  return m