Mutable defaults

[Chris Angelico]

On Thu, Feb 11, 2021 at 6:03 PM Ross Wilson <rzzzwilson at gmail.com> wrote:
>
> On Thu, 11 Feb 2564 BE at 12:52 Grant Edwards <grant.b.edwards at gmail.com>
> wrote:
>
> > On 2021-02-11, J. Pic <jpic at yourlabs.org> wrote:
> >
> > > I just meant removing the whole "default value mutating" story, not
> > > removing mutable variables. Really, I was wondering if there was a use
> > case
> > > where this actually turns to an advantage,
> >
> > I've seen people show how it can be used to provide function-scope
> > persistent storage -- the equivalent of declaring a static variable in
> > a C function. I don't think I've seen that done in the wild, though.
>
>
> Not sure this qualifies as "use in the wild", but the memoized naive
> Fibonacci is very nice when written this way:
>
> def fib(n, memo={0: 0, 1: 1}):
>     if n not in memo:
>         memo[n] = fib(n-1) + fib(n-2)
>     return memo[n]

Yep, that's a pretty elegant example of mutable defaults as statics.
In theory, you could use this for a Fibonacci-like sequence, just by
passing an appropriate memo dictionary; to make that work, the
recursive calls would have to pass that down the line.

# What comes next in the sequence 1 3 4 7 11?
fib(6, {1:1, 2:3})

But otherwise, the memo parameter isn't really being used as a
parameter, it's just a way to maintain state. It could just as easily
be a global, a closure cell, or anything else.

ChrisA