text: import os
  import abc
  
  from .py27compat import add_metaclass
  
  @add_metaclass(abc.ABCMeta)
  class Credential(object):
      """Abstract class to manage credentials
      """
  
      @abc.abstractproperty
      def username(self):
          return None
  
      @abc.abstractproperty
      def password(self):
          return None
  
  class SimpleCredential(Credential):
      """Simple credentials implementation
      """
  
      def __init__(self, username, password):
          self._username = username
          self._password = password
  
      @property
      def username(self):
          return self._username
  
      @property
      def password(self):
          return self._password
  
  class EnvironCredential(Credential):
      """Source credentials from environment variables.
         Actual sourcing is deferred until requested.
      """
  
      def __init__(self, user_env_var, pwd_env_var):
          self.user_env_var = user_env_var
          self.pwd_env_var = pwd_env_var
  
      def _get_env(self, env_var):
          """Helper to read an environment variable
          """
          value = os.environ.get(env_var)
          if not value:
              raise ValueError('Missing environment variable:%s' %env_var)
          return value
  
      @property
      def username(self):
          return self._get_env(self.user_env_var)
  
      @property
      def password(self):
          return self._get_env(self.pwd_env_var)
  