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To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: Closures in leu of pointers?
Date: Sat, 29 Jun 2013 12:01:47 +0200
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cts.private.yahoo@gmail.com wrote:

> I'd like to use closures to set allow a subroutine to set variables in its
> caller, in leu of pointers.

"leu"? Must be a Fench word ;)

> 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?

You picked the most obnoxious variable names I can think of, but the actual 
problem is simple:

Python statically determines the scope of a variable. If you rebind a name 
it becomes a local variable unless you explicitly declare it as global or -- 
in Python 3 -- as nonlocal. For example:

Wrong:

>>> def outer():
...     n = 0
...     def inner():
...             print(n)
...             n += 1
...     return inner
... 
>>> outer()()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in inner
UnboundLocalError: local variable 'n' referenced before assignment

With nonlocal declaration (Python 3 only):
>>> def outer():
...     n = 0
...     def inner():
...             nonlocal n
...             print(n)
...             n += 1
...     return inner
... 
>>> f = outer()
>>> f()
0
>>> f()
1
>>> f()
2

With a mutable variable as a pseudo-namespace (workaround for Python 2):

>>> def outer():
...     n = [0]
...     def inner():
...             print n[0]
...             n[0] += 1
...     return inner
... 
>>> f = outer()
>>> f()
0
>>> f()
1