[Python-Dev] PEP 289 - Generator Expressions - Let's Move Forward

Andrew Koenig ark-mlist at att.net
Mon May 3 15:19:39 EDT 2004

> > That leads me to wonder what exactly the rationale for generator
> > expressions is.  The PEP says that "time, clarity, and memory are
> > conserved by using an generator expression" but I can only see how
> > memory is conserved.  That is, I don't find them any easier to read than
> > list comprehensions and I don't understand the performance implications
> > very well.  It's not obvious to me that their faster.

> I've been skimming due to being out of town and catching up, but I
> haven't seen a direct response to Jeremy's question about the rationale.
> Jeremy, do you still want an answer?

I can see two potentially important cases where generator expressions win
big over list comprehensions:

	1) Where the code that is consuming the sequence yielded by the
generator expression terminates before consuming the entire sequence;

	2) Where the code that is consuming the sequence is an online
algorithm, and there is a potential delay between generating elements of the

It is easier to find an example of the second case than of the first:

	foo(bar(line) for line in sys.stdin)

If foo expects a generator as its input:

	def foo(x):
		for line in x:
			print line

then using a generator expression instead of a list comprehension will cause
the program to print each line of output after reading the corresponding
line of input, rather than consuming all the input and then printing all the

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