[IPython-dev] Magics that have return values, $$var=cmd simplification

Fernando Perez Fernando.Perez at colorado.edu
Tue Jan 24 12:53:54 CST 2006

Ville Vainio wrote:
> I figured we could implement a handy and intuitive way to grab
> both magic return values and system command output:
> var = %mymagic args a plenty
> var = !command args a plenty
> The regexp to catch these reliably is trivial (provided that var name
> is (\w+) and both are illegal python. Anyone have any
> doubt/objections? $$var = cmd syntax is there but it's not as easy to
> remember or obvious.

+1: it looks like it can be done safely, at least in single-line input.  I'd 
avoid this in continuation lines, because you could be in the middle of a 
string (triple-quoted).  In multiline input, people can always use

foo = ipmagic()

and friends.  Even for ipython, which tries as hard as possible to be 
convenient, correctness trumps convenience.

I'd suggest adding a test/ directory to ipython, where we can start at least 
storing primitive testing scripts.  For now, even if it's just plain files 
that we can copy/paste into a session to see whether we haven't totally blown 
something would be good.  I've many times in the past made a seemingly 
innocuous change to a regexp, just to find out that I broke something 
fundamental in input processing.

Having lines of valid python that we can just highlight and paste in would be 
good, and it would provide a minimal measure of safety when experimenting with 
extensions like this.

As time goes, this can grow into a proper unittest framework (for chainsaw, we 
WILL enforce unittest coverage for all code; no reason we can't start a little 



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