> > 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 <st...@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 > _______________________________________________ > Python-ideas mailing list > Python-ideas@python.org > https://mail.python.org/mailman/listinfo/python-ideas > Code of Conduct: http://python.org/psf/codeofconduct/ >
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