On Sun, Apr 13, 2014 at 5:28 AM, Joseph Martinot-Lagarde <joseph.martinot-lagarde@m4x.org> wrote:
The main use case is to be able to set a default value for indexing. I though about it after reading PEP 463 (Exception-catching expressions), this would be an alternative for one of the problem this PEP is trying to solve. Compare these solutions:
lst = [1, 2, 3] value = lst[3] Traceback (most recent call last): File "<stdin>", line 1, in <module> IndexError: list index out of range try: # Python .... value = lst[3] .... except IndexError: .... value = 0 value = lst[3] except IndexError: 0 # PEP 463 value = lst[3, default=0] # My proposal
An interesting idea, but that looks far too much like a function call. Square brackets and then more and more a function call inside... may fail the grit test. Part of the point of except expressions was that the implementation of __getitem__ wouldn't need to have two branches in it. Currently, you need to write __getitem__ (which raises an exception on finding a problem) plus something else, eg get(), which returns a default instead. By your proposal, both branches would go inside __getitem__, which means they could share code; but there still need to be two branches. That's two branches that need to be written, tested, etc, and if the author hasn't thought to handle default=, there's no way to get that functionality, same as if there's no get() method. And just as with the get method, there'll be an ad-hoc and fairly arbitrary puddle of names (some will go "default=", others will say that's way too long and go "def=", except that that's a keyword so they'll use "dflt=" or something...), unless there's a strong force pushing people to one consistent name. Terry's suggestion solves that part of it, but also restricts the feature to just default values; with a more general keyword argument system, it would make sense to also use this with __setitem__ and __delitem__, and I'm not sure how "default value" should be interpreted there. Which is better is, of course, a matter of opinion! :) ChrisA