To get everyone up to speed, Maya uses its own embedded scripting language called MEL - Maya Embedded Language. It's a good start for learning how Maya works behind the scenes, but it is rather rigid in its structure, and has a tendency to be very very verbose.
Newer versions of Maya have begun to include a Python runtime as well as a library which ports all (most? I'm not 100% sure) of Maya's MEL commands for use in Python. This essentially allows you to write MEL code while taking advantages of some of Python's nicer features: dynamically typed variables, more refined string manipulation, and not having to check every freaking line of code for the one semi-colon you missed that's causing your whole script to break. However, it still requires you to think largely in more of a MEL-type flow, which ends up feeling like a half-baked hybrid that nobody really would want to look at.
That's where PyMel is intended to come it. From what I gather, PyMel is designed as a more Python-y integration of Maya commands into Python. Honestly, I don't have enough experience to say for sure, but at first glance it does seem to be an improvement. I will add one caveat: it appears that others more experienced than I have a less-than-enthusiastic opinion of PyMel, primarily because they have found it to become excessively slow for large iterative tasks. However, since I'm writing small helper scripts rather than full-on tools, the speed differences have so far been indistinguishable. (As a final aside, a brief websearch indicates that PyMel is faster overall for scripting control, but it is faster to then switch to the Maya.Cmds functions for iterating large loops.)
Anyways, why is all this important? Because I'm a perfectionist and a minimalist (usually within reason). I like to do as much as I can with as little as I can get away with. For example, why write 5 lines of MEL code when I can do the same thing with 1 line of PyMel?
//Parent constrain lower-level control object's zero-group to higher-level control object //MEL Code string $OA_ctrl_parent_selection = `ls -sl`; string $parent_ctrl = $OA_ctrl_parent_selection; string $child_ctrl = $OA_ctrl_parent_selection; string $child_group = "GRP_" + $child_ctrl + "_zero"; parentConstraint -mo $parent_ctrl $child_group
//Parent constrain lower-level control object's zero-group to higher-level control object //Python/PyMel Code parentConstraint( ls(sl = 1) , "GRP_" + ls(sl = 1) + "_zero" , mo = 1 )
I'll be the first to admit that the PyMel snippet is much more difficult to decipher, but ultimately it's going onto my shelf as a button anyways. I'm also working on translating to PyMel the script I talked about in my previous post, but it's still a work in progress. In the end, though, I just really love writing these bits of automation to make my life easier. My rigging is already getting faster just by reducing the number of operations I have to execute to get the same results.