I agree with those who have suggested Panda3D -- it's a good choice for someone already experienced with Python wanting to make a 3d game, and has been used in a number of commercial games including at least one Massively Multiplayer Online Game from Disney (Pirates of the Caribbean Online). You can see the feature list here, and the documentation here. It's also both free and open-sourced, even for commercial use.
Note that Panda3D may have a steep learning curve if you're still a beginner, although it should be easy to pick up if you've had a reasonable amount of experience with Python.
Note: At the time of posting something is messed up in the forum stylesheet and links aren't standing out from normal text, so I've manually set the text of links to be bright blue as a work-around.