On Aug 28, 2:42 pm, Terry Reedy <tjre...@udel.edu> wrote:
> Carl Banks wrote:
> > I don't think it needs a syntax for that, but I'm not so sure a method
> > to modify a value in place with a single key lookup wouldn't
> > occasioanally be useful.
>
> Augmented assignment does that.
Internally uses two lookups, one for getting, and one for setting. I
think this is an unavoidable given Python's semantics. Look at the
traceback:
>>> def x():
... d['a'] += 1
...
>>> dis.dis(x)
2 0 LOAD_GLOBAL 0 (d)
3 LOAD_CONST 1 ('a')
6 DUP_TOPX 2
9 BINARY_SUBSCR
10 LOAD_CONST 2 (1)
13 INPLACE_ADD
14 ROT_THREE
15 STORE_SUBSCR
16 LOAD_CONST 0 (None)
19 RETURN_VALUE
> > As a workaround, if lookups are expensive,
>
> But they are not. Because (C)Python is heavily based on dict name lookup
> for builtins and global names and attributes, as well as overt dict
> lookup, must effort has gone into optimizing dict lookup.
The actual lookup algorithm Python dicts use is well-optimized, yes,
but the dict could contain keys that have expensive comparison and
hash-code calculation, in which case lookup is going to be slow.
Carl Banks
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