Terry J. Reedy <tjre...@udel.edu> added the comment:
In 2.x, map(None, 'abc', 'zyz') == [('a', 'z'), ('b', 'y'), ('c', 'z')], but
with the addition of zip, so that zip('abc', 'xyz') has the same result, we
deprecated that use of None to mean identity function.
For python-coded functions, a default is needed to make a keyword-only argument
optional, and preferred over use of *args for making positional arguments
optional. Unless I am missing something, a function can special-case 'key is
identity', to avoid overhead, just as well as it can special-case 'key is
None'. So rather than extend 'key=None', to me a kludge, I would rather
replace it with 'key=identity'. Both can be accepted during a deprecation
period.
For instance, after adding identity,
def nsmallest(n, iterable, key=identity):
...
if key is identity or key is None: # key in (identity, None)
result = min(it, default=sentinel)
...
Since no one need ever write key=None, explicit passing should be rare. It
seems to me that the main reason for the type declaration of key to include
None is so that the def statement itself passes a consistency check with the
None default. Once that is changed, most people should be able to use a simple
callable declaration. I am considering this for python-ideas.
Since the weekly issues list came out just 10 hours ago, I will not close this
yet, but I will if still open in couple of days and no coredev objections.
----------
nosy: +terry.reedy
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<https://bugs.python.org/issue34149>
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