Newbie: Why Does This Work This Way...Global Variables And The Signal Module....
markus at schabi.de
Fri Sep 14 12:05:26 CEST 2001
John Branthoover <jbranthoover at yahoo.com.NOSPAM> schrub:
> I have written a simple program to break out of a endless loop by
> hitting control C, see below. This program works OK but I don't
> understand why I have to define the global variable "Loop_Run" twice
> to get it to work. I thought that you had only to define a global
> variable once at the beginning of a program and that made it available
> to rest of the module.
The global statement is no definition in this way. All variables are
usually from their local scope, so if you do
then this sets the variable local to the current function. The global
statement just tells the byte-code compiler to use the global variable
loop_run during the current function whenever anything accesses
loop_run. Imagine this as some kind of "search redirection" which says
"don't use local scope, but global scope for this name".
So you have to tell this in every function and method you need it.
Also remember that the variable is created on the first write access to
it. A "definition" - as known from static languages - is not knowt to
Another point is that the python interpreter allows omitting the global
statement whenever the variable is only read and not written in the
function - because in this case, there can't be a local variable.
So you could rewrite your example as follows:
def signal_handler(signal, frame):
print 'Signal Handler',Loop_Run
print 'Loop is running because Loop_Run = ',Loop_Run
print 'Loop is stopped because Loop_Run = ',Loop_Run
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