def balance_map(source=None, **kwargs):

    # The source and/or kwargs should map currency name strings to

    # things you can pass to Decimal (a decimal string, an int, etc.)

    # This returns a dict that maps currency name strings to Amount instances.

    retval = {}

    if source is not None:

        retval.update((currency, Amount(number, currency))

                      for currency, number in source)

    if kwargs:

        retval.update(balance_map(kwargs.items()))

    return retval