def __iter__(self):
return iter(self.sources)<EOL>
Iterate over the sources in this registry.

        As ``Registry`` is a dict-like object, iterating over it only returns keys, ie classes.
        To iterate over values, ie sources, use for example ``.values()`` or ``.items()``.

        Returns
        -------
        iterator
            An iterator over the keys (ie classes) in the registry.

        Example
        -------

        >>> registry = Registry()
        >>> from datetime import date, datetime
        >>> registry.register(date, ['year', 'month', 'day'])
        >>> registry.register(datetime, ['day', 'timestamp'])
        >>> for klass in registry: print(klass)
        <class 'datetime.date'>
        <class 'datetime.datetime'>
        >>> for klass in registry.keys(): print(klass)
        <class 'datetime.date'>
        <class 'datetime.datetime'>
        >>> for source in registry.values(): print(source)
        <Source 'datetime.date'>
        <Source 'datetime.datetime'>
        >>> for klass, source in registry.items(): print('%s : %s' % (klass, source))
        <class 'datetime.date'> : <Source 'datetime.date'>
        <class 'datetime.datetime'> : <Source 'datetime.datetime'>