def __init__(self, fname, idx, key=None):
self.fname = fname<EOL>self.idx = idx<EOL>self.key = key<EOL>self._str = None<EOL>
Initialize a ``StepAddress`` instance.

:param fname: The file name of the YAML file.
:param idx: The index of the step within the file, or within
            the designated key of the file.
:param key: A key within the file.  If the file consists of a
            single list, this should be ``None`` (the
            default).