On Wed, Oct 21, 2020 at 06:17:27AM +1100, Chris Angelico wrote:
No; it can fail if the pattern actually rejects it. For instance, a pattern of "a{x}b{y}c" can match the string "a" but won't assign to y,
It won't assign to x either, if I'm understanding it correctly, since there is nothing following the 'a' that matches. Or will x get the empty string?
however it can't match the string "q" because that doesn't match. This behaviour of leaving variables unassigned happens ONLY if there's a partial match.
And just like with a regex, it would be easy enough to have a notation that demands end-of-string, and cannot match unless you reach that point in the pattern right as you reach the end of the string.
But you'll still get a partial match. Using regex syntax, if you try to match "spam {x} eggs$" (i.e. the prefix "spam", space, substring to be extracted, space, suffix "eggs", end of string) against "spam is great" you will get the partial match x="is". If you're trying to match the address "{num:d} Main Street {city}$" against "3.1415" you will get num=3. Maybe I'm missing something, but I don't see why this is desirable behaviour. [...]
Well... yes. And oddly enough, developing a scanning mini-language inspired by the formatting language is *exactly* what this proposal is, and has always been.
That's incorrect. The first post in this thread: https://mail.python.org/archives/list/python-ideas@python.org/thread/JEGSKOD... literally describes it as f-strings as the target. There's no hint that only a subset of f-string functionality will be accepted. Dennis says: "you can assign a string to an f-string" and proposes an exception: "ValueError: f-string assignment target does not match ..." I can't work out how to see messages in non-threaded date order on the website, but it looks to me like the first suggestion that this might not *literally* be f-strings comes about a dozen emails into the thread, and from you, not Dennis. The subject line doesn't say "Scanning mini-language inspired by f-strings", it says that f-strings should be allowed as an assignment target. I think that people can be forgiven for thinking that a thread that says it is about making f-strings assignment targets is about making f-strings assignment targets *wink* I think we have at least six mini-languages now? - regexes - string interpolation with % - template strings - format mini-language - f-strings - date/time format strings I think it would be quite an uphill battle to have a seventh built into the language. You're welcome to write a PEP :-) -- Steve