"attribute_1 = value_1, attribute_2 = value_2, ... "
where "attribute_n" specifies an attribute name, and the value to the right of each "=" sign should be a suitable textual representation of the value to be assigned. This value will be interpreted according to the attribute's data type.
The string supplied may also contain "printf"-style format specifiers, identified by "%" signs in the usual way. If present, these will be substituted by values supplied as additional optional arguments (using the normal "printf" rules) before the string is used.
AST A Library for Handling World Coordinate Systems in Astronomy