On Fri, Sep 18, 2020 at 10:53:57AM +1000, Chris Angelico wrote:
> On Fri, Sep 18, 2020 at 10:51 AM Steven D'Aprano <[email protected]> wrote:
> >
> > On Thu, Sep 17, 2020 at 11:09:35PM +1000, Chris Angelico wrote:
> >
> > > I've frequently yearned for an sscanf-like feature in Python. Usually
> > > I end up longhanding it with string methods, or else reaching for a
> > > regex, but neither of those is quite what I want. I'd prefer scanf
> > > notation to format strings, but either is acceptable.
> >
> > Why make this a syntactic feature when a scanf function would do?
> >
>
> Because a scanf function can't assign directly. In fact, the exact
> same issue that led to f-strings in the first place; there's no
> reliable way to embed the names into the format string without a lot
> of redundancy.
But that's a *separate problem*. Regexes can't assign directly either.
And we wouldn't want them to! (It's okay for a regex to have it's own
internal namespace, like named groups, but it shouldn't leak out into
the locals or globals.)
Extracting data from a string, like scanf, regexes, sed, awk, SNOBOL etc
sounds like a big win. Assignment should be a separate problem.
And at last I think I have thought of a use of dict unpacking I like. If
our scanf(pattern, target) function returns a dict of {name: value}
pairs, how do we apply it to locals?
target [, names] = **scanf(pattern, target)
where dict assignment matches assignment targets on the left with keys
in the dict. Acceptable target names are simple identifiers, dotted
names and subscripts:
spam, eggs.fried, cheese[0] = **{'cheese[0]': 3,
'spam': 1,
'eggs.fried': 2}
would do the obvious assignments. (I could live without the dotted names
and subscripts, if people don't like the additional complexity.)
Targets missing a key:value, or keys missing a target, would raise an
exception.
The bottom line here is that separation of concerns is a principle we
should follow. Text scanning and assignment are two distinct problems
and we should keep them distinct. This will allow us to pre-process the
pattern we want to match, and post-process the results of the scan, e.g.
spam, eggs, cheese = **(defaults | scanf(pattern, string))
We could have multiple scanners too, anything that returned a dict of
target names and values. We wouldn't need to build the scanner into the
interpreter, only the assignment syntax. The scanner itself is just a
function.
--
Steve
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