Functions as Objects, and persisting values
Please help me understand the mechanics of the following behavior.
def d():
header = 'I am in front of '
def e(something):
print header + something
return e
f = d()
f('this')
I am in front of this
del(d)
f('this')
I am in front of this
The way I understand it, function d is an object, as is e. However I
don't quite grok the exact relationship between e and d. Is e
considered to be a subclass of 'd', so that it has access to it's
parent's __dict__ object, in order to access the value of 'header'? Or
is this persistence managed in a different fashion?
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Re: Functions as Objects, and persisting values
Falcolas schrieb:
Please help me understand the mechanics of the following behavior.
def d():
header = 'I am in front of '
def e(something):
print header + something
return e
f = d()
f('this')
I am in front of this
del(d)
f('this')
I am in front of this
The way I understand it, function d is an object, as is e. However I
don't quite grok the exact relationship between e and d. Is e
considered to be a subclass of 'd', so that it has access to it's
parent's __dict__ object, in order to access the value of 'header'? Or
is this persistence managed in a different fashion?
The thing you observe here is a called a closure. It consists of the
local variables surrounding e. So as long as you keep a reference to e,
you keep one to the variables of d itself.
Diez
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Re: Functions as Objects, and persisting values
[snip] The thing you observe here is a called a closure. It consists of the local variables surrounding e. So as long as you keep a reference to e, you keep one to the variables of d itself. Diez More specifically though it keeps references to the requested variables only: def closed(): x = global_x y = Y_VAR def inner(): return y return inner class Destroyable(object): def __del__(self): print DESTROYED global_x = Destroyable() inner = closed() print inner() del global_x print inner() print HERE You will get: Y_VAR DESTROYED Y_VAR HERE If the entire dict of closed() was kept you would have seen: Y_VAR Y_VAR HERE DESTROYED Since closed hadn't been destroyed yet: thus there was only one reference remaining to global_x after closed() and inner() were called. Rich -- http://mail.python.org/mailman/listinfo/python-list
Re: Functions as Objects, and persisting values
Falcolas a écrit :
Please help me understand the mechanics of the following behavior.
def d():
header = 'I am in front of '
def e(something):
print header + something
return e
f = d()
f('this')
I am in front of this
del(d)
f('this')
I am in front of this
The way I understand it, function d is an object,
Right.
as is e.
Right.
However I
don't quite grok the exact relationship between e and d.
Is e
considered to be a subclass of 'd',
Nope.
so that it has access to it's
parent's __dict__ object, in order to access the value of 'header'? Or
is this persistence managed in a different fashion?
As Diez said, it's called a lexical closure. A lexical closure is a
function that captures and carries the lexical scope it was defined in
with it - it 'closes over' it's environnement. Each time you call d, it
returns a new function object (using the same code object) with a
different environnement. You'll find this environement in f.func_closure.
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