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Is there Python equivalent to Perl BEGIN{} block?

P: n/a
Hi all,

The subject says pretty much all, i would very appreciate an answer. I
tried to search the various forums and groups, but didn't find any
specific answer...

Thanks,
Alex.
Mar 12 '08 #1
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12 Replies


P: n/a
On Mar 12, 6:19 pm, Alex <alex.pul...@gmail.comwrote:
Hi all,

The subject says pretty much all, i would very appreciate an answer. I
tried to search the various forums and groups, but didn't find any
specific answer...

Thanks,
Alex.
No not really.

There are lots of other ways to structure a Python program though
which makes its omission hardly felt.

- Paddy.
Mar 12 '08 #2

P: n/a
The subject says pretty much all,

Given what I understand about the BEGIN block[1], this is how
Python works automatically:

bash$ cat a.py
print 'a1'
import b
print 'a2'

bash$ cat b.py
print 'b'

bash$ python a.py
a1
b
a2

However, the first import does win and Python caches
module-loading, so take care:

bash$ cat x.py
print 'one'
import a
print 'two'
import b
print 'three'

bash$ python x.py
one
a1
B
a2
two
three

-tkc

[1]
http://www.cs.cf.ac.uk/Dave/PERL/node133.html

Mar 12 '08 #3

P: n/a
On Wed, 12 Mar 2008 11:19:05 -0700, Alex wrote:
Hi all,

The subject says pretty much all
Only to people who know what the Perl BEGIN{} block means.

--
Steven

Mar 13 '08 #4

P: n/a
What do you need it for anyway? I just read about it and I think it's
useless
in python.

On Mar 13, 2008, at 1:03 AM, Steven D'Aprano wrote:
On Wed, 12 Mar 2008 11:19:05 -0700, Alex wrote:
>Hi all,

The subject says pretty much all

Only to people who know what the Perl BEGIN{} block means.

--
Steven

--
http://mail.python.org/mailman/listinfo/python-list
Mar 13 '08 #5

P: n/a
On Mar 12, 8:11 pm, Justus Schwabedal <justus.schwabe...@gmx.de>
wrote:
What do you need it for anyway? I just read about it and I think it's
useless
in python.

Perl, like Python, has a separate compilation and run times. One day,
someone who was trying to use Perl for something asked, "You know,
wouldn't it be neat-o if you could execute some Perl code *before* you
compiled the script?" And so (since that's usually enough reason to
add something to Perl) was borne the BEGIN block.

I believe the official rationale was that they wanted to add some
magic variables that affected Perl's compilation and couldn't do it
without a magic block that executed at compile time.

Python solved a similar problem by introducing __future__ imports.
Carl Banks
Mar 13 '08 #6

P: n/a
Alex wrote:
The subject says pretty much all, i would very appreciate an answer. I
tried to search the various forums and groups, but didn't find any
specific answer...
I'd like an answer to this, too. In Perl, I mostly used it for
one-liners, when a variable needed to be initialized to some non-default
value; for example, perl -ple 'BEGIN { $sum = 42 } $sum += $_'.
Mar 13 '08 #7

P: n/a
On Mar 13, 1:37 am, Carl Banks <pavlovevide...@gmail.comwrote:
On Mar 12, 8:11 pm, Justus Schwabedal <justus.schwabe...@gmx.de>
wrote:
What do you need it for anyway? I just read about it and I think it's
useless
in python.

Perl, like Python, has a separate compilation and run times. One day,
someone who was trying to use Perl for something asked, "You know,
wouldn't it be neat-o if you could execute some Perl code *before* you
compiled the script?" And so (since that's usually enough reason to
add something to Perl) was borne the BEGIN block.

I believe the official rationale was that they wanted to add some
magic variables that affected Perl's compilation and couldn't do it
without a magic block that executed at compile time.

Python solved a similar problem by introducing __future__ imports.

Carl Banks
And theres me thinking it was part of Perls AWK compatibility code.

- Paddy.
Mar 13 '08 #8

P: n/a
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...)
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.

Mar 13 '08 #9

P: n/a
On Mar 13, 7:02 am, Bruno Desthuilliers <bruno.
42.desthuilli...@wtf.websiteburo.oops.comwrote:
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...)
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
Mar 13 '08 #10

P: n/a
On Mar 13, 7:03 pm, Jonathan Gardner <jgard...@jonathangardner.net>
wrote:
On Mar 12, 6:37 pm, Carl Banks <pavlovevide...@gmail.comwrote:
<<Snip>>
>
And leave out the magical -p and -n. If you want to iterate through a
file, "for line in sys.stdin:".
Or better still:

import fileinput
for line in fileinput.input():
process(line)

- Paddy.
Mar 13 '08 #11

P: n/a
Paddy wrote:
On Mar 13, 7:03 pm, Jonathan Gardner <jgard...@jonathangardner.net>
wrote:
>On Mar 12, 6:37 pm, Carl Banks <pavlovevide...@gmail.comwrote:

<<Snip>>
>And leave out the magical -p and -n. If you want to iterate through a
file, "for line in sys.stdin:".

Or better still:

import fileinput
for line in fileinput.input():
process(line)
I write maybe a dozen one-liners a day. It's not practical to do this
with Python, or at least it's not nearly as convenient as Perl. I'm
fine with the magic. Abusing the magic is another story; Perl language
features are meant to be used in particular contexts, and BEGIN blocks
are rarely if ever necessary in Perl modules.
Mar 13 '08 #12

P: n/a
In article <hM******************************@comcast.com>,
Jeff Schwab <je**@schwabcenter.comwrote:
>
I write maybe a dozen one-liners a day. It's not practical to do this
with Python, or at least it's not nearly as convenient as Perl.
That is a defensible position, but my take is that writing the one-liners
in Python is more convenient than remembering enough Perl to make writing
one-liners useful. Especially when the one-liners often start expanding.
--
Aahz (aa**@pythoncraft.com) <* http://www.pythoncraft.com/

"All problems in computer science can be solved by another level of
indirection." --Butler Lampson
Mar 13 '08 #13

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