Hello, On Tue, 26 May 2020 08:37:59 +1000 Chris Angelico <[email protected]> wrote:
[] > > def foo(x := a + b) > > > > vs > > > > c = a + b > > def foo(x := c) > > > > can lead to different results. > > def foo(x = None): > if x is None: x = a + b > > c = a + b > def foo(x = None): > if x is None: x = c > > Is it surprising that these behave differently? You are refactoring > something from a late-bound default argument value into a global > variable. Surely it's obvious that it will now be evaluated once? Of course it will be obvious - you provided explicit control flow to make it work like that, and all that stays within the bounds of the existing semantics of the language. > > I'd suggest that people should love "explicit is better than > > implicit" principle of the language. > > Explicit meaning that you need to use a specific symbol that means > "this is to be late-bound"? No, it means "use explicit 'if' if you want to deal with mutable default". > Or explicit meaning "something that I like", as opposed to implicit > meaning "something that I don't like", which is how most people seem > to interpret that line of the Zen? I'd prefer to think in terms of implementation complexity. Implemented in adhoc way (and that's how things get implemented in C, in particular, in CPython), it will be quite a noticeable complexity up-glitch to function representation/implementation, and all it achieves is trading one confusion for another. (Well, for two others: why the heck there're 2 ways to define default args, which is to use when, and why one of them doesn't work across subexpression refactoring. Oh, and old confusion still stays with us. There's really no easy way to resolve the original confusion, short of banging mutable defaults. Which actually one that I'd like, because it would necessitate pushing "const" (currently, "Final") variable annotation down to the core of the language, for Python remains literally the last one which lacks it comparing to the competition.) > > ChrisA -- Best regards, Paul mailto:[email protected] _______________________________________________ Python-ideas mailing list -- [email protected] To unsubscribe send an email to [email protected] https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/[email protected]/message/BQAQ5YPU7T63BKRBDPSY3IDY5IVCM7WA/ Code of Conduct: http://python.org/psf/codeofconduct/
