Closures in leu of pointers?
Peter Otten
__peter__ at web.de
Sat Jun 29 06:01:47 EDT 2013
cts.private.yahoo at 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
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