It seems vims python sripting is designed to edit buffer and files rather than work nicely with vims registers. You can use some of the vim packages commands to get access to the registers but its not pretty.

My solution for creating a vim function using python that uses a register is something like this.

function printUnnamedRegister()
python >> EOF
print vim.eval('@@')
EOF

Setting registers may also be possible using something like

function setUnnamedRegsiter()
python >> EOF
s = "Some \"crazy\" string\nwith interesting characters"
vim.command('let @@="%s"' % myescapefn(s) )
EOF

However this feels a bit cumbersome and I'm not sure exactly what myescapefn should be. So I've never been able to get the setting version to work properly.

So if there's a way to do something more like

function printUnnamedRegister()
python >> EOF
print vim.getRegister('@')
EOF

function setUnnamedRegsiter()
python >> EOF
s = "Some \"crazy\" string\nwith interesting characters"
vim.setRegister('@',s)
EOF

Or even a nice version of myescapefn I could use then that would be very handy.