Closures in leu of pointers?

[cts.private.yahoo at gmail.com]

Hi,

I'd like to use closures to set allow a subroutine to set variables in its caller, in leu of pointers.  But I can't get it to work.  I have the following test pgm, but I can't understand its behaviour:

It uses a function p2() from the module modules.closure1b:

  def p2 (proc):
    proc ("dolly")

I thought the following worked like I expected it to:


from modules.closures1b import p2

def p1(msg1):
    msg3 = "world"
    print "p1: entered: ", msg1
    def p11(msg2):
        print "p11: entered: ", msg2
        print msg1 + msg2 + msg3
    print p2 (p11)

p1('hello')

$ python closures1c.py 
p1: entered:  hello
p11: entered:  dolly
hellodollyworld
None

In other words, p1() is passed "hello" for msg1, "world" goes to the local msg3 and then p11() is invoked out of a remote module and it can access not only its own argument (msg2) but also the variables local to p1(): "hellodollyworld".

But if I try to set the variable local to p1(), all of a sudden python seems to forget everything we agreed on. 

If I add this line to the script above:
        msg3 = "goodbye"
as follows:

from modules.closures1b import p2

def p1(msg1):
    msg3 = "world"
    print "p1: entered: ", msg1
    def p11(msg2):
        print "p11: entered: ", msg2
        print msg1 + msg2 + msg3
        msg3 = "goodbye"          # <- new
    print p2 (p11)

p1('hello')

then all of a sudden, I get this:

p1: entered:  hello
p11: entered:  dolly
Traceback (most recent call last):
  File "closures1c.py", line 13, in <module>
    p1('hello')
  File "closures1c.py", line 11, in p1
    print p2 (p11)
  File "/home/mellman/eg/python/modules/closures1b.py", line 2, in p2
    proc ("dolly")
  File "closures1c.py", line 9, in p11
    print msg1 + msg2 + msg3
UnboundLocalError: local variable 'msg3' referenced before assignment


Huh?  msg3 isn't more referenced than it was before!

Can anyone explain this to me?