Feature suggestion -- return if true

[Chris Angelico]

On Tue, Apr 12, 2011 at 10:27 AM, James Mills
<prologic at shortcircuit.net.au> wrote:
> This could be simplified to just:
>
> return expr or None
>
> And more to the point... If your calee is relying
> on the result of this function, just returning the
> evaluation of "expr" is enough.

I'm thinking here that that's not a solution; he'll have more code to
follow. An example of what I think he's trying to do:

def fac(n):
    # attempt to get from a cache
    return? cache[n]
    # not in cache, calculate the value
    ret=1 if n<=1 else fac(n-1)*n
    # and cache and return it
    cache[n]=ret; return ret

If the rest of the function can be implemented as an expression, it
might be possible to use:

return expr or other_expr

But in the example of a lookup cache, that wouldn't work so easily -
assignment isn't an expression. If 'x=y' had a value as it does in C,
the above function could become:

def fac(n):
    return cache[n] or (cache[n]=1 if n<=1 else fac(n-1)*n)

which is a reasonable one-liner, albeit not the most efficient
factorial implementation. Is there a simple and Pythonic way to do
this?

BTW, assume for the purposes of discussion that the return? expr is a
complex one, such that it's well worth evaluating only once (maybe
even has side effects).

Chris Angelico