Unfortunately the Python library that wraps GNU readline is blocking, therefore it won't work in an async server. But, readline does have an async API as well, so I set about investigating how to use it from Python. There seemed to be two choices. First was to write a C module that wraps the async features, and second was to use ctypes and call libreadline.so directly. Of course I googled a bit to see if anyone had done it and that is when I learned about ctypesgen. This is a nice little tool which takes a library and its include files, and spits out a Python module using ctypes that enables a Python application to use the same API as a C program would. ctypesgen: A Pure Python Wrapper Generator for ctypes.
So I tried it out like so:
python ctgen/ctypesgen.py -lreadline /usr/include/readline/*.h -o ctreadline.py
The end result was ctreadline.py, a Python module that was all ready for use. It only took a short while to read the libreadline docs and knock together this simple test program
runEnabled = True
nullstr = ctreadline.POINTER(ctreadline.c_char)()
# you must use == in this comparison because ln is a C object
if ln == None:
runEnabled = False
elif len(ln) > 0:
print " "
It doesn't do much, just echo back what you type, but it does do it asynchronously using "select" so it will be pretty straightforward to integrate in a command shell program and any async server based on select. Don't forget to try out your favourite readline features when you run it, things like Ctrl-R to search back, up-arrow and line editing.
The thing that took the longest to figure out was that you cannot compare the return value from libreadline in the usual way. I generally write if ln is None: but ctypes seems to return a different instance of None so you need to use the equals signs.
It can be hard to track down an API reference for libreadline and I ended up using the one for DOS here on Delorie's site.