':param env: The WSGI environment for the request.
:param expires: Unix timestamp as an int for when the URL
expires.
:param path: The path which is used for hashing.
:param scoped_keys: (key, scope) tuples like _get_keys() returns
:param request_method: Optional override of the request in
the WSGI env. For example, if a HEAD
does not match, you may wish to
override with GET to still allow the
HEAD.
:returns: a list of (hmac, scope) 2-tuples'

def _get_hmacs(self, env, expires, path, scoped_keys, request_method=None):

    if (not request_method):
        request_method = env['REQUEST_METHOD']
    return [(get_hmac(request_method, path, expires, key), scope) for (key, scope) in scoped_keys]
