[Python-ideas] PEP: Dict addition and subtraction

Brandt Bucher brandtbucher at gmail.com
Tue Mar 5 17:54:22 EST 2019


>
> Should our __sub__ behavior be the same...


Sorry, our "__isub__" behavior. Long day...

On Tue, Mar 5, 2019 at 2:47 PM Brandt Bucher <brandtbucher at gmail.com> wrote:

>  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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>>
>
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