This is actually applicable to any Python code, but is especially relevant to extensions that save data in Chimera sessions: the Python bytecode compiler has a limitation of a maximum of a 16 bit offset in a jump instruction. This means that you don't want to have 64K worth of characters in a single conditional block of code (i.e. if/else, loops, try/except). Otherwise you will get the somewhat opaque: SystemError: com_backpatch: offset too large So if you have data structures that may be potentially large when written to a session file, try to place their definitions in the session file outside of conditional blocks. --Eric Eric Pettersen UCSF Computer Graphics Lab pett@cgl.ucsf.edu http://www.cgl.ucsf.edu