> On Sat, Feb 6, 2021 at 5:21 PM Random832 <[email protected]> wrote:
> >
> > While we're on the subject of assignment expression limitations, I've
> > occasionally wanted to write something like
> >
> > try:
> > return a_dict[key]
> > except KeyError:
> > return (a_dict[key] := expression to construct value)
On Sat, Feb 6, 2021, at 01:26, Chris Angelico wrote:
>
> That's what the __missing__ method is for.
Requires a dict subclass
Requires that the value be determinable from the key alone [rather than any
other variables available in the function containing the above code]
On Sat, Feb 6, 2021, at 01:50, Brendan Barnwell wrote:
> You can already do that with `return a_dict.setdefault(key,
> your_expression_here)`. If the expression is expensive to evaluate you
> can use a short-circuiting conditional expression to guard it.
How exactly would you use a short-circuiting conditional expression to do this?
If you're suggesting `... if key not in a_dict else ...` this creates a race
condition, and also involves checking the key in the dictionary twice.
Perhaps a constructdefault method could be added to dict, broadly equivalent to
def constructdefault(self, func):
try:
return self[key]
except KeyError:
return (self[key] := func())
you could use self[key] = value = func(); return value; and the same in the
original [not part of dict class] snippet I posted above, but the point is this
seems so much like exactly the sort of use case that := is intended for, that
it comes across as weird that it's not usable.
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