Re: [Numpy-discussion] recarray field names

[Erin S. <eri...@gm...>]

Nice.  Python decides to compare with the keys and not the values.

The possibilities for obfuscation are endless.

On 3/15/06, Fernando Perez <Fer...@co...> wrote:
> Erin Sheldon wrote:
>
> > Yes, I see, but I think you meant
> >
> >     if name in t.dtype.fields.keys():
>
> No, he really meant:
>
> if name in t.dtype.fields:
>
> dictionaries are iterators, so you don't need to construct the list of ke=
ys
> separately.  It's just a redundant waste of time and memory in most cases=
,
> unless you intend to modify the dict in your loop, case in which the iter=
ator
> approach won't work and you /do/ need the explicit keys() call.
>
> In addition
>
> if name in t.dtype.fields
>
> is faster than:
>
> if name in t.dtype.fields.keys()
>
> While both are O(N) operations, the first requires a single call to the h=
ash
> function on 'name' and then a C lookup in the dict's internal key table a=
s a
> hash table, while the second is a direct walkthrough of a list with
> python-level equality testing.
>
> In [15]: nkeys =3D 1000000
>
> In [16]: dct =3D dict(zip(keys,[None]*len(keys)))
>
> In [17]: time bool(-1 in keys)
> CPU times: user 0.01 s, sys: 0.00 s, total: 0.01 s
> Wall time: 0.01
> Out[17]: False
>
> In [18]: time bool(-1 in dct)
> CPU times: user 0.00 s, sys: 0.00 s, total: 0.00 s
> Wall time: 0.00
> Out[18]: False
>
>
> In realistic cases for your original question you are not likely to see t=
he
> difference, but it's always a good idea to be aware of the performance
> characteristics of various approaches.  For a different problem, there ma=
y
> well be a real difference.
>
> Cheers,
>
> f
>