global and None
ptmcg at users.sourceforge.net
Tue Dec 23 16:13:10 CET 2003
"Francis Avila" <francisgavila at yahoo.com> wrote in message
news:vug3s0pa1q1sa3 at corp.supernews.com...
> Leo Yee wrote in message ...
> >My purpose is to init g. This is very useful when we cannot construct a
> >global variable.
> Like when? And why?
One reason is to do late construction/initialization, or lazy
initialization. If an object is very time-consuming to construct, don't
create it at import time, wait until it is actually used. Once created,
save it in a global so that you don't have to re-create it every time.
This is sort of a quick-and-dirty singleton pattern. I do a very similar
thing to construct my pyparsing grammar *once*, then save it in the global
and re-reference it on subsequent calls.
sqlGrammar = None
if sqlGrammar is None:
# construct SQL grammar - this only needs to happen once
sqlGrammar = CaselessLiteral("select") + <blah, blah, blah>
def test( sqlstring ):
return getSQLGrammar().parseString( sqlstring )
test( "SELECT * from XYZZY, ABC" )
test( "select * from SYS.XYZZY" )
test( "Select A from Sys.dual" )
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