I'd like to propose adding `append` and `extend` methods to dicts which behave like `__setitem__` and `update` respectively, except that they raise an exception (KeyError?) instead of overwriting preexisting entries.
Very often I expect that the key I'm adding to a dict isn't already in it. If I want to verify that, I have to expand my single-line assignment statement to 3-5 lines (depending on whether the dict and key are expressions that I now need to assign to local variables). If I don't verify it, I may overwrite a dict entry and produce silently wrong output. The names `append` and `extend` make sense now that dicts are defined to preserve insertion order: they try to append the new entries, and if that can't be done because it would duplicate a key, they raise an exception. In case of error, `extend` should probably leave successfully appended entries in the dict, since that's consistent with list.extend and dict.update. The same methods would also be useful on sets. Unfortunately, the names make less sense. -- Ben _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
