On 2015-08-02 16:37, Eric V. Smith wrote:
On 8/1/2015 1:43 PM, Eric V. Smith wrote:
On 7/25/2015 3:55 PM, Eric V. Smith wrote:
In trying to understand the issues for a PEP, I'm working on a sample implementation. There, I've just disallowed concatentation entirely. Compared to all of the other issues, it's really insignificant. I'll put it back at some point.
I'm basically done with my implementation of f-strings.
Here's another issue. I can't imagine this will happen often, but it should be addressed. It has to do with literal expressions that begin with a left brace.
For example, this expression:
{x: y for x, y in [(1, 2), (3, 4)]} {1: 2, 3: 4}
If you want to put it in an f-string, you'd naively write:
f'expr={{x: y for x, y in [(1, 2), (3, 4)]}}' 'expr={x: y for x, y in [(1, 2), (3, 4)]}'
But as you see, this won't work because the doubled '{' and '}' chars are just interpreted as escaped braces, and the result is an uninterpreted string literal, with the doubled braces replaced by undoubled ones.
There's currently no way around this. You could try putting a space between the left braces, but that fails with IndentationError:
f'expr={ {x: y for x, y in [(1, 2), (3, 4)]}}' File "<fstring>", line 1 {x: y for x, y in [(1, 2), (3, 4)]} ^ IndentationError: unexpected indent
Why is there an IndentationError? It's an expression, not a statement, so leading spaces should be ignored. They're not at the Python prompt, but, then, that accepts both statements and expressions.
In the PEP I'm going to specify that leading spaces are skipped in an expression. So that last example will now work:
f'expr={ {x: y for x, y in [(1, 2), (3, 4)]}}' 'expr={1: 2, 3: 4}'
Note that the right braces in that last example aren't interpreted as a doubled '}'. That's because the first one is part of the expression, and the second one ends the expression. The only time doubling braces matters is inside the string literal portion of an f-string.
I'll reflect this "skip leading white space" decision in the PEP.