Extend is an API-only function which binds a new external function as a command into the PyMOL scripting language. In other words, when you write a function and want PyMOL to recognize the new command, you extend the command into PyMOL. Once extended, the function name is recognized like other function names (example below). Typically, extend is the last line of a PyMOL script.
cmd.extend(string name,function function)
def foo(moo=2): print moo cmd.extend('foo',foo)
The following would now work within PyMOL:
PyMOL>foo 2 PyMOL>foo 3 3 PyMOL>foo moo=5 5 PyMOL>foo ? Usage: foo [ moo ]
For security reasons, new PyMOL commands created using "extend" are not saved or restored in sessions.
"extend" used in selections
"extend" can also be used in selection statements to grow a selection based on covalent bonds. This behavior is illustrated on residue 64 of PDB file 1KAO where we initially select one atom and then "extend" that selection by one and three covalent bonds.
|select (resi 64 and n. cz)||select (resi 64 and n. cz) extend 1||select (resi 64 and n. cz) extend 3|