[Tutor] List comprehension + lambdas - strange behaviour

[spir ☣]

On Thu, 6 May 2010 22:15:34 +0100
"Alan Gauld" <alan.gauld at btinternet.com> wrote:

> As others have pointed out you are returning a reference not a value.

Yes. (I have said that, too.) But still there is a mystery for me. Better explained byt the following:

x = 0 ; print id(x)	# an address
def f() : print x	# 0
x = 1 ; print id(x)	# another one
f()			# 1

This shows, I guess, that the reference of the upvalue x is *not* an address. But the key (maybe the name itself ?) used by python to lookup a symbol's value, in a given scope, at runtime. Indeed f must find its upvalue in the global scope. Note the scope must also be referenced:

def f():
	# not the global scope
	x = 0
	def g():
		print x
	x = 1
	return g
# global scope
f()()			# 1

I guess the above example also shows that upvalues can be "finalised", since here the scope is lost xwhen f runs.

Does anyone know if this reasoning is correct, and how this is done?

All of this mechanics looks very complicated. I would be happy with setting func attributes like x here as func attributes directly & explicitely:

def f():
	def g():
		print g.x
	g.x = 1

... which are still modifiable --explicitely.

Denis
________________________________

vit esse estrany ☣

spir.wikidot.com