CDMS classes are grouped into internal, interface, and user classes. Only user classes are meant to be used directly. An interface class defines the common interface of the classes derived from it. For simplicity this document describes the interface classes, and only discusses the derived user classes in the few cases where the interface differs.
Transient classes are not associated with a container such as a Dataset or CdmsFile. They are `in-memory' classes. In contrast, persistent classes such as Axis and DatasetVariable are associated with a container. Modification of an instance of the class persists after the program ends, that is, I/O is generated to a file.
Note that the TransientVariable class is derived from class array in module MA, the masked array class. Masked arrays may be thought of as containing: an array of data, a mask of ones and zeros, and a "fill value". Complete documentation is available in the Numerical Python package at http://sourceforge.net/projects/numpy.