readline trick needed

[Steven D'Aprano]

I'm working with the readline module, and I'm trying to set a key 
combination to process the current command line by calling a known 
function, *and* enter the command line.

Something along the lines of:

* execute function spam() in some context where it can access 
  the current command line as a string
* enter the command line

Function spam() may or may not modify the command line.

Here is what I have got so far: I can discard the current line and call a 
function:

readline.parse_and_bind(r'"\C-p": "%cspam()\n"' % 0x15)  # ^U

binds ctrl-P to the key combinations `ctrl-U spam() Enter`, which clears 
the command line before entering spam().

If I leave out the ctrl-U, I'll get a SyntaxError or other exception, 
e.g. command line `x = 123` gets transformed into `x = 123spam()`.


This is not suitable:

readline.parse_and_bind(r'"\C-p": "; spam()\n"')

because it changes the command line. It's okay for spam() itself to 
modify the command line, but the key binding should not.

I tried to do this:

readline.parse_and_bind(r'"\C-p": "\nspam()\n"')

but it gives me a segmentation fault, which is a little less helpful than 
I had expected.

This Stackoverflow question suggests that what I want is not possible in 
vanilla Python:

http://stackoverflow.com/questions/11680356


but I'm a stubborn guy and I have not given up yet. Any suggestions?


(P.S. I'm aware of IPython, I want to get this working in the standard 
CPython interpreter.)


-- 
Steven