Make NTEnum useful

[mdavidsaver]

A rather involved change to the NT wrapper interface in order to make NTEnum useful (#25).

Mainly a replacement/extension of the client side wrapper functions with a stateful wrap/unwrap object. This allows p4p.nt.enum.NTEnum to cache a choices list, allowing for index lookup in wrap() and unwrap() where it might not otherwise be available.

This is really a workaround for the race conditions inherent in the way PVA protocol and the NTEnum are defined.

The p4p.nt.enum.ntenum wrapper acts like an integer, with an additional member choice containing the string (if known).

There are still some TODOs here. I wonder if the wrapper object should be compatible with str?

@tynanford Would this work for you? Improvements?

Server side usage would be:

from p4p.nt import NTEnum
from p4p.server.thread import SharedPV
pv = SharedPV(nt=NTEnum(), initial={'choices':['zero','one'], 'index':1})
assert pv.current()==1, pv.current() # compare ntenum with int
print pv.current() # prints "one\n"

pv.post(0) # as integer
assert pv.current()==0 # compare ntenum with int

pv.post(1) # as string
assert pv.current()==1

# change choices list
pv.post({'choices':['A','B','C'], 'index':2})
assert pv.current()==2

Client side usage against QSRV w/ an mbboRecord

record(mbbo, "select") {
    field(ZRVL, "0")
    field(ONVL, "1")
    field(ZRST, "Zero")
    field(ONST, "One")
}

would be

from p4p.client.thread import Context
ctxt=Context('pva')
print repr(ctxt.get('select'))
# "ntenum(0, Zero)\n"
ctxt.put('select', 1)
print repr(ctxt.get('select'))
# "ntenum(1, One)\n"
ctxt.put('select', 'Zero')
print repr(ctxt.get('select'))
# "ntenum(0, Zero)\n"

Documentation of this new NT interface is tbd.

[tynanford]

Yes this is great! Everything is working.

[mdavidsaver]

Rebased and merged as d60cbd7