[Python-ideas] function defaults and an empty() builtin

[Bruce Leban]

On Fri, May 20, 2011 at 12:27 PM, Masklinn <masklinn at masklinn.net> wrote:

>
> Absolutely, but the mutation of the default parameters seems to be the main
> problem (historically): it's a memory leak, and it's a global data
> corruption,
> where modifying a provided parameter is a local data corruption (unless the
> object passed in is global of course).
>
>
Agreed, but that wasn't how I interpreted Jack's question. I think there are
two issues (with the first one the one that I think Jack was targeting):

(1) I want to make sure a parameter is immutable so I don't accidentally
change it (and none of the functions I call can do that). Akin to declaring
a parameter const in C-like languages. For example, is_ip_blocked(ip_address,
blocked_ip_list). (That's not just ip_address in blocked_ip_list if the list
contains CIDR addresses.) We could add code to make the values immutable or
use annotations:

    @const
    def func(x : const, y : const = []):
        pass

Personally, I like declaring the contract that a parameter is not being
modified explicitly and I would like a shallow freeze() function.

(2) The gotcha that the default value is the same value every time rather
than a new value. Lot's of ways to deal with this but none of them work
without educating people how the feature works. For example:

    @copy
    def func(x : copy, y : copy = []):
        pass


--- Bruce
Latest blog post: http://www.vroospeak.com Your social security number is a
very poor password
Learn how to hack web apps: http://j.mp/gruyere-security
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