Re: [Python-ideas] Python-ideas Digest, Vol 131, Issue 106
On 31.10.2017 8:37, python-ideas-request@python.org wrote:
Thunderbird jerked on me big time. It never did anything like this before! Switched off Digest mode, individual messages aren't so complicated. def single(i): try: v =i.next() except StopIteration: raise ValueError('No items') try: i.next() except StopIteration: return v else: raise ValueError('More than one item') print single(name for name in('bob','fred') if name=='bob') -- Regards, Ivan
On Tue, Oct 31, 2017 at 5:18 PM, Ivan Pozdeev via Python-ideas <python-ideas@python.org> wrote:
Thanks :) One small change: If you use next(i) instead of i.next(), your code should work on both Py2 and Py3. But other than that, I think it's exactly the same as most people would expect of this function. ChrisA
On Tue, Oct 31, 2017 at 06:02:34PM +1100, Chris Angelico wrote:
Seems like an awfully complicated way to do by hand what Python already does for you with sequence unpacking. Why re-invent the wheel?
Not me. As far as I can tell, that's semantically equivalent to: def single(i): result, = i return result apart from slightly different error messages. -- Steve
On Tue, Oct 31, 2017 at 6:46 PM, Steven D'Aprano <steve@pearwood.info> wrote:
I saw the original code as being like the itertools explanatory functions - you wouldn't actually USE those functions, but they tell you what's going on when you use the simpler, faster, more compact form. ChrisA
On Tue, Oct 31, 2017 at 10:01 AM, Chris Angelico <rosuav@gmail.com> wrote:
I wonder if that's more easily understood if you write it along these line(s): (the_bob,) = (name for name in ('bob','fred') if name=='bob') People need to learn about how to make a 1-tuple quite early on anyway, and omitting the parentheses doesn't really help there, AFAICT. Then again, the idiom looks even better when doing a, b = find_complex_roots(polynomial_of_second_order) Except of course that I couldn't really come up with a good example of something that is expected to find exactly two values from a larger collection, and the students are already coming into the lecture hall. Or should it be (a, b,) = find_complex_roots(polynomial_of_second_order) ? ––Koos -- + Koos Zevenhoven + http://twitter.com/k7hoven +
On Tue, Oct 31, 2017 at 11:24 AM, Petr Viktorin <encukou@gmail.com> wrote:
Maybe it's just me, but somehow that list-like syntax as an assignment target feels wrong in somewhat the same way that (1, 2).append(3) does. ––Koos PS. In your previous email, something (your email client?) removed the vertical line from the quoted Chris's email, so it looks like just an indented block. I wonder if a setting could fix that. -- + Koos Zevenhoven + http://twitter.com/k7hoven +
On Tue, Oct 31, 2017 at 12:18 PM Koos Zevenhoven <k7hoven@gmail.com> wrote:
Off topic: why can't we simply allow something like this: (the_bob) = (name for name in ('bob','fred') if name=='bob') Why does Python treat the parenthesis at the LHS as grouping parens? operators are not allowed anyway; (a + (b + c)) = [1] is syntax error. Currently (x) = 1 works, but I can't see why should it. Elazar
On Tue, Oct 31, 2017 at 10:31:50AM +0000, אלעזר wrote:
Off topic: why can't we simply allow something like this:
(the_bob) = (name for name in ('bob','fred') if name=='bob')
Parens don't make a tuple. They are just for grouping. If you want a tuple, you need a comma: the_bob, = ... with or without the parens. It would be terribly surprising if (x) was a sequence on the left hand side but not on the right hand side of an assignment.
Why does Python treat the parenthesis at the LHS as grouping parens? operators are not allowed anyway; (a + (b + c)) = [1] is syntax error.
a, (b, c), d = [1, "xy", 2]
Why shouldn't it? Its just a trivial case of the fact that the left hand side can be certain kinds of expressions, some of which require parens: (spam or ham)[x] = value There are lots of possible expressions allowed on the LHS, and no good reason to prohibit (x) even though its pointless. -- Steve
On Tue, Oct 31, 2017 at 5:18 PM, Ivan Pozdeev via Python-ideas <python-ideas@python.org> wrote:
Thanks :) One small change: If you use next(i) instead of i.next(), your code should work on both Py2 and Py3. But other than that, I think it's exactly the same as most people would expect of this function. ChrisA
On Tue, Oct 31, 2017 at 06:02:34PM +1100, Chris Angelico wrote:
Seems like an awfully complicated way to do by hand what Python already does for you with sequence unpacking. Why re-invent the wheel?
Not me. As far as I can tell, that's semantically equivalent to: def single(i): result, = i return result apart from slightly different error messages. -- Steve
On Tue, Oct 31, 2017 at 6:46 PM, Steven D'Aprano <steve@pearwood.info> wrote:
I saw the original code as being like the itertools explanatory functions - you wouldn't actually USE those functions, but they tell you what's going on when you use the simpler, faster, more compact form. ChrisA
On Tue, Oct 31, 2017 at 10:01 AM, Chris Angelico <rosuav@gmail.com> wrote:
I wonder if that's more easily understood if you write it along these line(s): (the_bob,) = (name for name in ('bob','fred') if name=='bob') People need to learn about how to make a 1-tuple quite early on anyway, and omitting the parentheses doesn't really help there, AFAICT. Then again, the idiom looks even better when doing a, b = find_complex_roots(polynomial_of_second_order) Except of course that I couldn't really come up with a good example of something that is expected to find exactly two values from a larger collection, and the students are already coming into the lecture hall. Or should it be (a, b,) = find_complex_roots(polynomial_of_second_order) ? ––Koos -- + Koos Zevenhoven + http://twitter.com/k7hoven +
On Tue, Oct 31, 2017 at 11:24 AM, Petr Viktorin <encukou@gmail.com> wrote:
Maybe it's just me, but somehow that list-like syntax as an assignment target feels wrong in somewhat the same way that (1, 2).append(3) does. ––Koos PS. In your previous email, something (your email client?) removed the vertical line from the quoted Chris's email, so it looks like just an indented block. I wonder if a setting could fix that. -- + Koos Zevenhoven + http://twitter.com/k7hoven +
On Tue, Oct 31, 2017 at 12:18 PM Koos Zevenhoven <k7hoven@gmail.com> wrote:
Off topic: why can't we simply allow something like this: (the_bob) = (name for name in ('bob','fred') if name=='bob') Why does Python treat the parenthesis at the LHS as grouping parens? operators are not allowed anyway; (a + (b + c)) = [1] is syntax error. Currently (x) = 1 works, but I can't see why should it. Elazar
On Tue, Oct 31, 2017 at 10:31:50AM +0000, אלעזר wrote:
Off topic: why can't we simply allow something like this:
(the_bob) = (name for name in ('bob','fred') if name=='bob')
Parens don't make a tuple. They are just for grouping. If you want a tuple, you need a comma: the_bob, = ... with or without the parens. It would be terribly surprising if (x) was a sequence on the left hand side but not on the right hand side of an assignment.
Why does Python treat the parenthesis at the LHS as grouping parens? operators are not allowed anyway; (a + (b + c)) = [1] is syntax error.
a, (b, c), d = [1, "xy", 2]
Why shouldn't it? Its just a trivial case of the fact that the left hand side can be certain kinds of expressions, some of which require parens: (spam or ham)[x] = value There are lots of possible expressions allowed on the LHS, and no good reason to prohibit (x) even though its pointless. -- Steve
participants (6)
-
Chris Angelico -
Ivan Pozdeev -
Koos Zevenhoven -
Petr Viktorin -
Steven D'Aprano -
אלעזר