On Nov 30, 2019, at 13:19, Soni L. <[email protected]> wrote:
>
> I have a lot of code that looks like this:
>
> def filter(self, it, defs):
> for x in it:
> for y in _match_helper(self.key, defs, x[0]):
> yield (y, x[1])
Try destructuring it:
for key, value in it:
And now you can use key instead of x[0] and value instead of x[1]. The loop
target in a for loop is like a target in an assignment statement, and can do
most of the same things. (This doesn’t work for all bindings—you can’t
destructure an argument in a parameter list, or from spam import * as first,
*rest—but as a first approximation it works anywhere it wouldn’t be confusing,
which is usually all you need to remember.)
> It'd be quite nice if dict.items() returned a namedtuple so all these x[0],
> x[1], el[0], el[1], etc would instead be x.key, x.value, el.key, el.value,
> etc.
I think dict would want to implement it as a C-level structseq (like stat
results) instead of a Python-level namedtuple for convenience and performance,
but that’s not a big issue.
Anyway, I think this would be a good idea. But I’m not sure it’s feasible
The problem is that items() is part of a protocol that many types implement,
including many third-party types (sortedcontainers.SortedDict, pyrsistent.PMap,
pyobjc.NSDictionary, java.collections.Map, whatever the type is for attribute
mappings in beautifulsoup, etc., not to mention project-internal types). A
whole lot of code is written to work with ,“any mapping”, and all of that code
will have to keep using [0] and [1] instead of .key and .value until every
mapping type it might be handed has been updated. (That includes the
constructors of most of those types, which can handle any object with an
items() method just like dict can.)
You could get about 75% of the way there pretty easily. This
structseq/namedtuple/whatever type could be given a name, and
collections.abc.ItemsView could iterate values of that type, so everyone who
inherits the items method from the Mapping mixin or builds their own items view
that inherits the ItemsView mixin gets the new behavior. Together with types
that inherit itend from dict, or delegate it to dict without transforming the
results, that covers a lot of mapping types. But it still doesn’t cover all of
them.
And I don’t think even a future or deprecation schedule would help here. You
could document that, starting in 3.12, items must be an 2-item sequence with
key and value attributes that are equal to the first and second element instead
of just being an sequence with two values. But that’s not something the ABC can
test for you, so I don’t think it would have the desired result of pushing the
whole ecosystem to change in 4.5 years; people would have to keep using [0] and
[1] for many years to come if they wanted to work with all mapping types.
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