[Python-Dev] Seeming unintended difference between list comprehensions and generator expressions... (original) (raw)

Josiah Carlson josiah.carlson at gmail.com
Sat Feb 21 01🔞05 CET 2009

On Fri, Feb 20, 2009 at 3:14 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

Josiah Carlson wrote:

The behavior of 3.0 WRT list comprehensions behaving the same way as generator expressions is expected, but why generator expressions (generally) don't keep a reference to the class scope during execution seems to be unintended. It's intended. While arguably not ideal, it would require some pretty major changes to the lexical scoping rules to make them behave any differently. The translation of (i*i for i in x) is conceptually along the lines of: def ge(arg): for i in arg: yield i*i = ge(x) Similarly, a 3.x list comprehension [i*i for i in x] is very roughly translated as: def lc(arg): result = [] for i in arg: result.append(i*i) return result = lc(x)

I was under the impression that in 3.x, it was equivalent to list() . Which would explain the difference between 2.6 and 3.0.

Like any function scope inside a class namespace, the body of a genexp (and, in 3.x, comprehension) doesn't have direct access to the class namespace because classes don't play any part in the lexical scoping rules.

Indeed, though I had thought (if only briefly ;) ) that when executing non-definitions in the class body, it would behave similar to the a more or less equivalent function-based class factory

def makeclass(*bases): ... def make_class(fcn): ... dict = fcn() ... return type(object)(fcn.name, bases, dict) ... return make_class ... @makeclass(object) ... def foo(): ... x = {} ... x.update((i, x.get(i, None)) for i in xrange(10)) ... return locals() ... foo <class '__main__.foo'> foo.x {0: None, 1: None, 2: None, 3: None, 4: None, 5: None, 6: None, 7: None, 8: None, 9: None}

But I was wrong ;)

Thank you for the help :)

Basically, if a generator or 3.x comprehension needs access to a value from a containing class scope anywhere other than the outermost iterator, then it needs to be put into a temporary function and given the extra value as an argument:

.>> class C: ... x = {} ... def initconstants(d, itr): ... d.update((i, d.get(i, None)) for i in itr) ... initconstants(x, range(10)) ... del initconstants ... .>> C.x {0: None, 1: None, 2: None, 3: None, 4: None, 5: None, 6: None, 7: None, 8: None, 9: None} (in this toy case, of course, it would be simpler to make the temporary function create and return the constants dictionary, but the above approach with multiple arguments being passed in applies more generally when you need to access multiple existing values from the class scope) Cheers, Nick. -- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia ---------------------------------------------------------------

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