':param environ: The environment variables (defaults to
``os.environ`` if no value is provided).
:param mapping: An optional mapping of variable names to
environment variable names.  Use this if you want to
change the mapping of access_key->AWS_ACCESS_KEY_ID, etc.
The dict can have up to 3 keys: ``access_key``, ``secret_key``,
``session_token``.'

def __init__(self, environ=None, mapping=None):

    if (environ is None):
        environ = os.environ
    self.environ = environ
    self._mapping = self._build_mapping(mapping)
