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-   -   getting equal behavior for scripts and modules ? (http://www.velocityreviews.com/forums/t701215-getting-equal-behavior-for-scripts-and-modules.html)

Stef Mientki 10-11-2009 05:50 PM

getting equal behavior for scripts and modules ?
 
hello,

I do agree that circular references should preferable be avoided.

In languages like Delphi, you get an error message, trying to use
circular references,
but solving them in large programs with a lot of history can be very
painful.

Now I finally (after 2 years) knowing there's a difference between
modules and scripts,
I want to guarantee that I always get the same functional behavior.

I found 2 solutions to realize the above.

=== solution 1 ===
Inserting a launcher into the IDE,
so instead of running the application as a script,
the file will always be executed as a module.
"""
== Launcher ==
instead of
if __name__ == '__main__' :
define a function
def main () :
and start this Launcher with the first parameter being the name of the
module
Launcher <module to be tested> <other arguments>
"""

import sys
__My_Main_Application = __import__ ( sys.argv[1] )

if 'main' in dir ( __My_Main_Application ) :
__My_Main_Application.main ()



=== solution 2 ===
Prevent execution of the code in this file if the file is ran as a script.
if __name__=='__main__':
import os, sys

# determine the name of myself
a = sys._getframe().f_code.co_filename
X = os.path.splitext ( os.path.split(a)[1] ) [0]

#import myself as 'ME'
ME = __import__ ( X )

# run some code in myself
ME.functional_code ()

# prevent that the code below is executed,
# ( for the second time )
# if this file is used as a script
sys.exit()

print 'One time import code'
def functional_code () :
print 'Functional Code'



any comment ?
thanks,
Stef Mientki

Steven D'Aprano 10-11-2009 10:06 PM

Re: getting equal behavior for scripts and modules ?
 
On Sun, 11 Oct 2009 19:50:31 +0200, Stef Mientki wrote:

> Now I finally (after 2 years) knowing there's a difference between
> modules and scripts,
> I want to guarantee that I always get the same functional behavior.


You are confused. Scripts *are* modules. What makes a script a script is
that it is executed from the shell:

$ python ~/scripts/helloworld.py
Hello World.
$

The difference isn't between files which *are* scripts and files which
*are* modules, but between modules which do something useful when *run as
a script* versus modules which don't. Recent versions of Python even
include a switch -m that searches the sys.path for the named module and
runs the module's .py file as a script.

You don't get the same behaviour between *running* a module and
*importing* a module because they do different thing. When you say
"import module" inside the Python environment, the module goes through
the import machine. When you say "python script.py" in the shell, it
doesn't.

When you import a module, Python looks to see if it has already been
imported, and if it has, it returns the module object in the cache. If
not, then it does a whole lot of magic which includes executing the code
inside the module. This means that modules get executed only once in any
session (unless you remove it from the cache).

When you run a module as a script from the shell, it doesn't go through
the import machinery, it doesn't get looked up in the cache, and it gets
executed every time.

The consequence of this is that if you run a module as a script, it gets
executed. If, in the process of being executed, it imports itself
(directly or indirectly), the import machinery has to run it again. Hence
the module gets executed twice.


> I found 2 solutions to realize the above.


Over-engineered overly-complicated non-solution to a non-problem.

The simplest solution is as follows: code that must always be executed
goes in the body of the module. Code that *only* gets executed when
running as a script goes under a test:



if __name__ == '__main__':
# running as a script
# code goes here
pass

That code will *only* run when running as a script, and not when you
import the module. Everything outside such a test will always run. If you
don't want that distinction, then don't use such a test.



--
Steven


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