I really like the symmetry of this approach.
On Thu, Sep 17, 2020 at 8:37 AM Dennis Sweeney <sweeney.dennis...@gmail.com>
wrote:
> TL;DR: I propose the following behavior:
>
> >>> s = "She turned me into a newt."
> >>> f"She turned me into a {animal}." = s
> >>> animal
> 'newt'
>
> >>> f"A {animal}?" = s
> Traceback (most recent call last):
> File "<pyshell#2>", line 1, in <module>
> f"A {animal}?" = s
> ValueError: f-string assignment target does not match 'She turned me
> into a newt.'
>
> >>> f"{hh:d}:{mm:d}:{ss:d}" = "11:59:59"
> >>> hh, mm, ss
> (11, 59, 59)
>
> === Rationale ===
>
> Part of the reason I like f-strings so much is that they reduce the
> cognitive overhead of reading code: they allow you to see *what* is
> being inserted into a string in a way that also effortlessly shows
> *where* in the string the value is being inserted. There is no need to
> "paint-by-numbers" and remember which variable is {0} and which is {1}
> in an unnecessary extra layer of indirection. F-strings allow string
> formatting that is not only intelligible, but *locally* intelligible.
>
> What I propose is the inverse feature, where you can assign a string
> to an f-string, and the interpreter will maintain an invariant kept
> in many other cases:
>
> >>> a[n] = 17
> >>> a[n] == 17
> True
>
> >>> obj.x = "foo"
> >>> obj.x == "foo"
> True
>
> # Proposed:
> >>> f"It is {hh}:{mm} {am_or_pm}" = "It is 11:45 PM"
> >>> f"It is {hh}:{mm} {am_or_pm}" == "It is 11:45 PM"
> True
> >>> hh
> '11'
>
> This could be thought of as analogous to the c language's scanf
> function, something I've always felt was just slightly lacking in
> Python. I think such a feature would more clearly allow readers of
> Python code to answer the question "What kinds of strings are allowed
> here?". It would add certainty to programs that accept strings,
> confirming early that the data you have is the data you want.
> The code reads like a specification that beginners can understand in
> a blink.
>
>
> === Existing way of achieving this ===
>
> As of now, you could achieve the behavior with regular expressions:
>
> >>> import re
> >>> pattern = re.compile(r'It is (.+):(.+) (.+)')
> >>> match = pattern.fullmatch("It is 11:45 PM")
> >>> hh, mm, am_or_pm = match.groups()
> >>> hh
> '11'
>
> But this suffers from the same paint-by-numbers, extra-indirection
> issue that old-style string formatting runs into, an issue that
> f-strings improve upon.
>
> You could also do a strange mishmash of built-in str operations, like
>
> >>> s = "It is 11:45 PM"
> >>> empty, rest = s.split("It is ")
> >>> assert empty == ""
> >>> hh, rest = rest.split(":")
> >>> mm, am_or_pm = s.split(" ")
> >>> hh
> '11'
>
> But this is 5 different lines to express one simple idea.
> How many different times have you written a micro-parser like this?
>
>
> === Specification (open to bikeshedding) ===
>
> In general, the goal would be to pursue the assignment-becomes-equal
> invariant above. By default, assignment targets within f-strings would
> be matched as strings. However, adding in a format specifier would
> allow the matches to be evaluated as different data types, e.g.
> f'{foo:d}' = "1" would make foo become the integer 1. If a more complex
> format specifier was added that did not match anything that the
> f-string could produce as an expression, then we'd still raise a
> ValueError:
>
> >>> f"{x:.02f}" = "0.12345"
> Traceback (most recent call last):
> File "<pyshell#2>", line 1, in <module>
> f"{x:.02f}" = "0.12345"
> ValueError: f-string assignment target does not match '0.12345'
>
> If we're feeling adventurous, one could turn the !r repr flag in a
> match into an eval() of the matched string.
>
> The f-string would match with the same eager semantics as regular
> expressions, backtracking when a match is not made on the first
> attempt.
>
> Let me know what you think!
> _______________________________________________
> Python-ideas mailing list -- python-ideas@python.org
> To unsubscribe send an email to python-ideas-le...@python.org
> https://mail.python.org/mailman3/lists/python-ideas.python.org/
> Message archived at
> https://mail.python.org/archives/list/python-ideas@python.org/message/JEGSKODAK5MCO2HHUF4555JZPZ6SKNEC/
> Code of Conduct: http://python.org/psf/codeofconduct/
>
>
--
CALVIN SPEALMAN
SENIOR QUALITY ENGINEER
cspea...@redhat.com M: +1.336.210.5107
[image: https://red.ht/sig] <https://red.ht/sig>
TRIED. TESTED. TRUSTED. <https://redhat.com/trusted>
_______________________________________________
Python-ideas mailing list -- python-ideas@python.org
To unsubscribe send an email to python-ideas-le...@python.org
https://mail.python.org/mailman3/lists/python-ideas.python.org/
Message archived at
https://mail.python.org/archives/list/python-ideas@python.org/message/DQPXMODQQE662IXCRK4GTX4XTHC5W5C4/
Code of Conduct: http://python.org/psf/codeofconduct/
