scope of generators, class variables, resulting in global na

dontspamleo dontsendleospam at gmail.com
Thu Feb 25 23:02:31 CET 2010


Hi Arnaud et al,

Here is the link to the bug report from which the discussion in PEP
289 was extracted:

http://bugs.python.org/issue872326

It looks like they were fixing a bunch of bugs and that this
discussion was one of the many in that thread.

Here is another post which points to the core of the issue: early/late
binding. It is also pointed to in PEP 289.

http://mail.python.org/pipermail/python-dev/2004-April/044555.html

Here is Guido's rationale (his text is the items (a) and (b) below. My
comments follow.

(a) I find it hard to defend automatic variable capture given Python's
    general late-binding semantics

MY COMMENTS: That is a good point. There are however exceptions to the
"general late-binding semantics" (GLBS) in Python. The outer loop does
bind early in this case in fact. A point to consider along with GLBS
is the unintuitive early bind of the external iterator vs late bind of
the internal iterator. That is a function not of early vs. late
binding, but of the current translation of generator expressions to
generator functions. Using Arnaud's approach would fix that and make
the code IMO so more pythonic.

(b) I believe that use cases needing early binding are uncommon and
    strained: they all involve creating a list of generator
    expressions, which IMO is a pretty unusual thing to do

MY COMMENTS: This is actually a pretty common use case. Any iteration
over objects of arity 2 or greater (A matrix, a table, a tree searched
breadthwise under some data structures, etc.) can conveniently and
cleanly be expressed using a generator expression.

I'd also add that perhaps the title of Guido's post: "Generator
Expressions - Let's Move Forward" may have unintentionally discouraged
people from reexamining the issue.

I would like to see this decision revisited. Obviously before spending
any time on this I'd like to gauge if there is further interest. Am I
off the mark? Maybe there is a technical misunderstanding on my part.
If there are more people affected (Since the last week I found some
other postings here and on other lists and blogs) who can make a clear
case, then what is the next step?

Cheers,
Leo.



More information about the Python-list mailing list