Is there Python equivalent to Perl BEGIN{} block?

[Arnaud Delobelle]

On Mar 14, 10:37 pm, Alex <alex.pul... at gmail.com> wrote:
> On Mar 13, 6:21 pm, Carl Banks <pavlovevide... at gmail.com> wrote:
>
> > On Mar 13, 7:02 am, Bruno Desthuilliers <bruno.
>
> > 42.desthuilli... at wtf.websiteburo.oops.com> 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
> C).
>
>
>
> > > > 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
>
> Hi,
>
> Maybe i was a little bit unclear... I meant that i wanted to do
> something like this:
>
> #!usr/bin/env python
>
> check_env()
>
> 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?

why not:

try:
    from subprocess import *
except ImportError:
    sys.exit('No subprocess module :(')

# Rest of module

--
Arnaud