Writing clients in Python

RTS2 provides three external bindings, which can be used by clients to interact with the RTS2 environment:

1. RTS2 protocol
2. XML/RPC protocol
3. JSON

JSON, standing for JavaScript Object Notation, is probably the simplest to use. To demonstrate the simplicity of its use:

# import library with RTS2 functions
import login
# for sleep function
import time
# sign on to JSON server
l = login.Login()
# this will pop out dialog box, asking user for credentials
l.signon()
# now set C0 temperature to -20
l.jsonProxy().setValue('C0','CCD_SET',-20)
temp = float(l.jsonProxy().getValue('C0','CCD_TEMP'))
while abs(float(temp + 20) > 0.5:
    print 'temperature',temp,'air temperature',l.jsonProxy().getValue('C0','CCD_AIR')
    # refresh cache..can be avoided by having timer running update, similar to loop which can be found in rts2ui
    l.jsonProxy().executeCommand('C0','info')
    l.refresh_cache()
    temp = float(l.jsonProxy().getValue('C0','CCD_TEMP'))
    # wait for 5 seconds
    time.sleep(5)
print 'Desired temperature reached',l.jsonProxy().getValue('C0','CCD_TEMP')

This code demonstrates how you can write a simple client in Python, which does things similar to C++ client provided in the INDI tutorial. It illustrates the extreme simplicity of the external JSON API (in this case further masked by the Python library). After establishing connection, your client communicates via a kept-alive HTTP connection - so you don't pay the extra price for opening and closing communication.

JSON is much more dense then XML, so messages are shorter, and parameters are passed as URL parameters - you can talk to the server with a web-browser (or wget command), which is handy for debugging.

Errors are propagated as standard Python errors, and as they are not caught, they will ordinarily cause a client to terminate with a traceback writen to standard output. If you wish to catch possible errors, all you have to do is to add try-except blocks.

This also provides basic security, as a login is required. To disable external access entirely, just kill rts2-xmlrpcd, which provides the service.