Is there Python equivalent to Perl BEGIN{} block?

Alex alex.pulver at
Fri Mar 14 23:37:59 CET 2008

On Mar 13, 6:21 pm, Carl Banks <pavlovevide... at> wrote:
> On Mar 13, 7:02 am, Bruno Desthuilliers <bruno.
> 42.desthuilli... at> wrote:
> > Alex a écrit :
> > (sni)
> > > First of all thanks all for answering!
> > > I have some environment check and setup in the beginning of the code.
> > > I would like to move it to the end of the script.
> > Why ? (if I may ask...)

Sure, because of a readability (similar to function declarations in

> > > But I want it to
> > > execute first, so the script will exit if the environment is not
> > > configured properly.
> > If you want some code to execute first when the script/module is loaded,
> > then keep this code where it belongs : at the beginning of the script.
> I concur with Bruno's recommendation: stuff you want to do first
> should come first in the script.  Things like BEGIN blocks hurt
> readability because you can't identify where execution begins without
> reading the whole file.
> Having said that, one thing that often happens in Python scripts is
> that all the functions are defined first, then the script logic
> follows.  So you could put the meat of your script in a function, then
> the "BEGIN" stuff after that functions:
> def run_script():
>     #
>     # script contained in this long function
>     #
> # Then test preconditions here...
> if os.environ["HELLO"] != "WORLD":
>     sys.exit(2)
> # Then call the run_script functions
> run_script()
> But having said THAT, I don't recommend you do that with
> preconditions.  If the script has a quick early exit scenario, you
> really ought to put that near the top, before the function
> definitions, to clearly show to a human reader what is necessary to
> run the script.
> Carl Banks


Maybe i was a little bit unclear... I meant that i wanted to do
something like this:

#!usr/bin/env python


from subprocess import *

class MyClass:
   # Class definition

def check_env():
   # Code

if __name__ == "__main__":
   # Script logic

The thing is, as i saw, that Python doesn't recognize the
"check_env()" function before it reaches the "def" statement.

I need the check to be done before the subprocess import, because our
customers use different Python versions, some of them do not have
subprocess module. So i want to exit if i see that Python version
being used doesn't have that module.

The solution to that problem with what you suggested could be wrapping
the subprocess import with function, am i correct?

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