On 2015-08-10 20:23, Guido van Rossum wrote:
On Mon, Aug 10, 2015 at 8:49 PM, Eric V. Smith <eric@trueblade.com <mailto:eric@trueblade.com>> wrote:
On 08/10/2015 02:44 PM, Yury Selivanov wrote: > On 2015-08-10 2:37 PM, Eric V. Smith wrote: >> This is why I think PEP-498 isn't the solution for i18n. I'd really like >> to be able to say, in a debugging context: >> >> print('a:{self.a} b:{self.b} c:{self.c} d:{self.d}') >> >> without having to create locals to hold these 4 values. > > Why can't we restrict expressions in f-strings to > attribute/item getters? > > I.e. allow f'{foo.bar.baz}' and f'{self.foo["bar"]}' but > disallow f'{foo.bar(baz=something)}'
It's possible. But my point is that Barry doesn't even want attribute/item getters for an i18n solution, and I'm not willing to restrict it that much.
I also don't want to tie this closely to i18n. That is (still) very much a wold of its own.
What I want with f-strings (by any name) is a way to generalize from print() calls with multiple arguments. We can write
print('Todo:', len(self.todo), '; busy:', len(self.busy))
but the same thing is more awkward when you have to pass it as a single string to a function that just sends one string somewhere. And note that the above example inserts a space before the ';' which I don't really like. So it would be nice if instead we could write
print(f'Todo: {len(self.todo)}; busy: {len(self.busy)}')
which IMO is just as readable as the multi-arg print() call[1], and generalizes to other functions besides print().
In fact, the latter form has less punctuation noise than the former -- every time you insert an expression in a print() call, you have a quote+comma before it and a comma+quote after it, compared to a brace before and one after in the new form. (Note that this is an argument for using f'{...}' rather than '\{...}' -- for a single interpolation it's the same amount of typing, but for multiple interpolations, f'{...}{...}' is actually shorter than '\{...}\{...}', and also the \{ part is ugly.)
Anyway, this generalization from print() is why I want arbitrary expressions. Wouldn't it be silly if we introduced print() today and said "we don't really like to encourage printing complicated expressions, but maybe we can introduce them in a future version"... :-)
Continuing the print()-generalization theme, if things become too long to fit on a line we can write
print('Todo:', len(self.todo), '; busy:', len(self.busy))
Can we allow the same in f-strings? E.g.
print(f'Todo: {len(self.todo) }; busy: {len(self.busy) }')
or is that too ugly? It could also be solved using implicit concatenation, e.g.
print(f'Todo: {len(self.todo)}; ' f'busy: {len(self.busy)}')
[1] Assuming syntax colorizers catch on.
I'd expect f'...' to follow similar rules to '...'. You could escape it: print(f'Todo: {len(self.todo)\ }; busy: {len(self.busy)\ }') which would be equivalent to: print(f'Todo: {len(self.todo) }; busy: {len(self.busy) }') or use triple-quoted a f-string: print(f'''Todo: {len(self.todo) }; busy: {len(self.busy) }''') which would be equivalent to: print(f'Todo: {len(self.todo)\n }; busy: {len(self.busy)\n }') (I think it might be OK to have a newline in the expression because it's wrapped in {...}.)