On Dec 31, 2007 2:08 PM, Odalrick <[EMAIL PROTECTED]> wrote: > On 31 Dec, 18:22, Arnaud Delobelle <[EMAIL PROTECTED]> wrote: > > On Dec 31, 10:58 am, Odalrick <[EMAIL PROTECTED]> wrote: > > > > > On 30 Dec, 17:26, George Sakkis <[EMAIL PROTECTED]> wrote: > > > > > > On Dec 29, 9:14 pm, bukzor <[EMAIL PROTECTED]> wrote: > > > > > > > Here's the answer to the > > > > > question:http://www.python.org/doc/faq/general/#why-are-default-values-shared-... > > > > > > > It looks like Guido disagrees with me, so the discussion is closed. > > > > > > Note that the FAQ mainly explains *what* happens, not *why* was this > > > > decision taken. Although it shows an example where "this feature can > > > > be useful", it's neither the only way to do it nor is memoization as > > > > common as wanting fresh default arguments on every call. > > > > > I'm surprised noone has said anything about the why of default > > > mutables. I think it is becasue it isn't easy to do it an other way. > > > > [...] > > > > There is an easy enough way: evaluate default values when the function > > is called rather than when it is defined. This behaviour comes with > > its own caveats as well I imagine, and it's not 'as easy' to implement > > as the current one. > > > > Adding overhead to *all* function calls, even the ones without mutable > defaults. That doesn't sound like an attractive tradeoff. >
And also removing the only way you can currently do early binding in Python. I agree that it's a gotcha, but unless someone comes up with an answer to the following questions, I'll stick with the status quo (Note that this is not blind Python group-think as a previous poster implied, but a pragmatic decision that this is the most practical solution): a) If we don't evaluate default arguments at function compilation, when do we do it? b) If you do it at call time, how do you implement early binding? c) If you want to introduce new syntax for the current behavior, what is it and can you justify it? d) What are the performance implications of your proposal versus the current behavior? Note that the desired behavior can be implemented under the current behavior, at the expense of verbosity - using factories and sentinel values as the default arguments, and then expanding them in the function. It's not possible to implement the current behavior of early-bound arguments if default arguments are evaluated with every call. This alone is a good reason to keep the current behavior until someone actually has a good alternative that covers the current use cases and isn't just upset by the behavior. -- http://mail.python.org/mailman/listinfo/python-list
