[Python-ideas] PEP: Dict addition and subtraction

[Brandt Bucher]

>
>  These semantics are intended to match those of update as closely as
> possible. For the dict built-in itself, calling keys is redundant as
> iteration over a dict iterates over its keys; but for subclasses or other
> mappings, update prefers to use the keys method.
>
> The above paragraph may be inaccurate. Although the dict docstring states
> that keys will be called if it exists, this does not seem to be the case
> for dict subclasses. Bug or feature?
>

>>> print(dict.update.__doc__)
D.update([E, ]**F) -> None.  Update D from dict/iterable E and F.
If E is present and has a .keys() method, then does:  for k in E: D[k] =
E[k]
If E is present and lacks a .keys() method, then does:  for k, v in E: D[k]
= v
In either case, this is followed by: for k in F:  D[k] = F[k]

It's actually pretty interesting... and misleading/wrongish. It never says
that keys is *called*... in reality, it just checks for the "keys" method
before deciding whether to proceed with PyDict_Merge or PyDict_MergeFromSeq2.
It should really read more like:

D.update([E, ]**F) -> None.  Update D from dict/iterable E and F.
If E is present, has a .keys() method, and is a subclass of dict, then
does:  for k in E: D[k] = E[k]
If E is present, has a .keys() method, and is not a subclass of dict, then
does:  for k in E.keys(): D[k] = E[k]
If E is present and lacks a .keys() method, then does:  for k, v in E: D[k]
= v
In either case, this is followed by: for k in F:  D[k] = F[k]

Should our __sub__ behavior be the same (i.e., iterate for dict subclasses
and objects without "keys()", otherwise call "keys()" and iterate over
that)? __iadd__ calls into this logic already. It seems to be the most
"natural" solution here, if we desire behavior analogous to "update".

Brandt

On Fri, Mar 1, 2019 at 8:26 AM Steven D'Aprano <steve at pearwood.info> wrote:

> Attached is a draft PEP on adding + and - operators to dict for
> discussion.
>
> This should probably go here:
>
> https://github.com/python/peps
>
> but due to technical difficulties at my end, I'm very limited in what I
> can do on Github (at least for now). If there's anyone who would like to
> co-author and/or help with the process, that will be appreciated.
>
>
> --
> Steven
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