[Python-Dev] Possible resolution of generator expression variablecapture dilemma (original) (raw)

Tim Peters tim.one at comcast.net
Thu Mar 25 00:30:44 EST 2004

BTW, have we had an example of a generator expression where late binding was semantically necessary? The only one I recall was a strained one along the lines of:

sqs = (sq(i) for i in range(n))

def sq(i):
    return i**2

Late binding was needed then because the generator expression referenced a name not yet bound when the g-e was evaluated.

Is the popular plan now that this "will work", but provided that "n" is bound before the g-e is evaluated? That is, this works:

n = 10
sqs = (sq(i) for i in range(n))
def sq(i): ...
print list(sqs)

but this doesn't work:

sqs = (sq(i) for i in range(n))
n = 10  # swapped with the line above
def sq(i): ...
print list(sqs)

and this uses different bindings for the two lexical instances of "n" in the g-e:

n = 10
sqs = (sq(i+n) for i in range(n)  # 10 used in range(n), 20 in i+n
n = 20
def sq(i): ...
print list(sqs)

?

Not wondering how bad it can get, just wondering how bad it starts .

More information about the Python-Dev mailing list