On 10/11/2013 8:08 AM, Franck Ditter wrote:
In article <5257c3dd$0$29984$c3e8da3$54964...@news.astraweb.com>,
Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info> wrote:
On Fri, 11 Oct 2013 10:14:29 +0300, Jussi Piitulainen wrote:
Roy Smith writes:
In article <m2a9ihxf3a....@cochabamba.vanoostrum.org>,
Piet van Oostrum wrote:
I usually say that a closure is a package, containing a function with
some additional data it needs. The data usually is in the form of
name bindings.
That's pretty close to the way I think about it. The way it was
originally described to me is, "A closure is a function bundled up with
it's arguments".
Really? It should be more like "a function bundled up with some other
function's arguments" and even more like "a function bundled up with
bindings for its free variables".
Closures have nothing to do with *arguments*. A better definition of a
closure is that it is a function together with a snapshot of the
environment it was called from.
def func(arg):
y = arg + 1
def inner():
return y + 1000
return inner
f = func(1)
Maybe a better example of closure would be (just for the nonlocal) :
def fib() :
(a,b) = (0,1)
a,b = 0,1 is the same thing.
a and b are separate local names and are in no sense a 'pair'.
def producer() :
nonlocal a,b # Python 3
old = a
(a,b) = (b,a+b)
return old
return producer
f = fib()
[f() for i in range(10)]
[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
At this point, f is a closure. It needs to know the value of y (not the
argument to func) in order to work, and the implementation is to store
that information inside f.func_closure (or f.__closure__ in Python 3).
The part of the calling environment which is saved is y
Shouldn't it be the (a,b) pair here ? But :
f.__closure__[0].cell_contents # access to what ?
55
Shouldn't cell_contents keep the current (a,b) pair, a part of the snapshot of
the creation environment (private variables of the closure) ?
Instead it seems to returns only a (which is the next production)...
Look as f.__closure__[1] (.cell_contents) for b.
--
Terry Jan Reedy
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