[Python-Dev] listcomps vs. for loops

[Walter Dörwald]

Guido van Rossum wrote:

> [...]
> The variable of a for *statement* must be accessible after the loop
> because you might want to break out of the loop with a specific
> value.  This is a common pattern that I have no intent of breaking.
> So it can't introduce a new scope; then it might as well keep the last
> value assigned to it.
> 
> List comprehensions and generator expressions don't have 'break'.
> (You could cause an exception and catch it, but it's not a common
> pattern to use the control variable afterwards -- only the debugger
> would need access somehow.)

How about an until keyword in generator expressions:

sum(len(line) for line in file if not line.startswith("#") until not 
line.strip())

Would the order of "if" and "until" be significant?

And we could have accumulators first() and last():

def first(it):
	return it.next()

def last(it):
	for value in it:
		pass
	return value

first(line for line in file if line.startswith("#"))

if not last(file):
	# last line not terminated

Bye,
    Walter Dörwald