[Python-Dev] What about PEP 299?

[Nick Coghlan]

Guido van Rossum wrote:
> On 3/28/06, Charles Cazabon <python at discworld.dyndns.org> wrote:
>> It might be worth instead adding an option flag to the executable that implies
>> "from the loaded module, run __main__() with sys.argv as its argument(s)", so
>> the user can get this behaviour with `python -X somemodule.py`.
> 
> You can do "python -x somemodule" as long as somemodule.py uses the if
> __name__=='__main__' convention. What does your proposal add?

FWIW, you can already (Python 2.4) do something like:

------- x.py ---------

import sys

if __name__ == __main__:
     del sys.argv[0]                 # Get rid of reference to ourselves
     mod_name = sys.argv[0]          # First arg is now module to be run
     mod = __import__(mod_name)      # Run the top level module code
     main = mod.__main__             # Grab the main function
     sys.modules["__main__"] = mod   # Make that module the __main__ one
     try:
         sys.argv[0] = mod.__file__  # Try to set argv[0] properly
     except AttributeError:
         pass
     sys.exit(main(*sys.argv))       # Run the function

-------------------------

Put that in site-packages and "python -mx somemodule" will do exactly as 
Charles describes.

Getting it to work with a filename instead of a module name would be a bit 
trickier, but not a lot.

However, I think PEP 299 is mainly a holdover from C/C++/Java where the top 
level of a module is a play area for the compiler that the runtime never 
really gets to see. I know I found PEP 299 appealing when I first seriously 
started using Python, but the appeal faded over time as I got used to the idea 
of being able to have control logic at the top level of a module (to the point 
where the idea is now thoroughly *un*appealing).

PEP 299's other 'use case' (trying to run another program's main function from 
within the current program) seems like a recipe for disaster - far better to 
use the subprocess module instead (since, strangely enough, application 
initialisation code has this tendency to assume it has sole control of the 
interpreter).

Cheers,
Nick.


-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
---------------------------------------------------------------
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