From: Ryan Davis <ryand-ruby@...>
Date: 2010-11-10T09:24:26+09:00
Subject: Re: Analyzer for errors in code ?


On Nov 9, 2010, at 14:56 , David Unric wrote:

> Unfortunately none of above mentioned or linked tools detected the use 
> of unassigned variable. For an illustration what does pylint for the 
> equivalent code in python:
> 
> _____ snip ________________________________________
> import sys
> 
> if __name__ == "__main__":
>    my_msg = 'Hello'
>    if sys.argv[-1] == 'doit':
>        print mymsg
>    else:
>        print 'Nope'
> 
> ~$ pylint -E test.py
> No config file found, using default configuration
> ************* Module test
> E:  6: Undefined variable 'mymsg'

In python, 'mymsg' _has_ to be a variable. This isn't true in ruby. It can be a variable OR a method call. In the case of the latter, we don't know if it is valid or not without evaluating.

in ruby, 'def x; mymsg; end' (the most boiled down version of your example) looks like this internally:

% echo 'def x; mymsg; end' | parse_tree_show 
s(:defn,
 :x,
 s(:args),
 s(:scope, s(:block, s(:call, nil, :mymsg, s(:arglist)))))

Ruby parsed the code and decided that mymsg must be a method call. It is determined at runtime (since everything is late bound) who (if anyone) implemented the method and if not, it goes to method_missing.

Python simply doesn't have this flexibility. 'x' is a variable and 'x()' is a call. AFAIK, there is no "__" hook equivalent to method_missing in python... But it has been a while for me.