[Numpy-discussion] New numpy functions: filled, filled_like

Olivier Delalleau shish@keba...
Mon Jan 14 10:15:30 CST 2013


2013/1/14 Matthew Brett <matthew.brett@gmail.com>:
> Hi,
>
> On Mon, Jan 14, 2013 at 9:02 AM, Dave Hirschfeld
> <dave.hirschfeld@gmail.com> wrote:
>> Robert Kern <robert.kern <at> gmail.com> writes:
>>
>>>
>>> >>> >
>>> >>> > One alternative that does not expand the API with two-liners is to let
>>> >>> > the ndarray.fill() method return self:
>>> >>> >
>>> >>> >   a = np.empty(...).fill(20.0)
>>> >>>
>>> >>> This violates the convention that in-place operations never return
>>> >>> self, to avoid confusion with out-of-place operations. E.g.
>>> >>> ndarray.resize() versus ndarray.reshape(), ndarray.sort() versus
>>> >>> np.sort(), and in the broader Python world, list.sort() versus
>>> >>> sorted(), list.reverse() versus reversed(). (This was an explicit
>>> >>> reason given for list.sort to not return self, even.)
>>> >>>
>>> >>> Maybe enabling this idiom is a good enough reason to break the
>>> >>> convention ("Special cases aren't special enough to break the rules. /
>>> >>> Although practicality beats purity"), but it at least makes me -0 on
>>> >>> this...
>>> >>>
>>> >>
>>> >> I tend to agree with the notion that inplace operations shouldn't return
>>> >> self, but I don't know if it's just because I've been conditioned this way.
>>> >> Not returning self breaks the fluid interface pattern [1], as noted in a
>>> >> similar discussion on pandas [2], FWIW, though there's likely some way to
>>> >> have both worlds.
>>> >
>>> > Ah-hah, here's the email where Guide officially proclaims that there
>>> > shall be no "fluent interface" nonsense applied to in-place operators
>>> > in Python, because it hurts readability (at least for Dutch people
>>> > ):
>>> >   http://mail.python.org/pipermail/python-dev/2003-October/038855.html
>>>
>>> That's a statement about the policy for the stdlib, and just one
>>> person's opinion. You, and numpy, are permitted to have a different
>>> opinion.
>>>
>>> In any case, I'm not strongly advocating for it. It's violation of
>>> principle ("no fluent interfaces") is roughly in the same ballpark as
>>> np.filled() ("not every two-liner needs its own function"), so I
>>> thought I would toss it out there for consideration.
>>>
>>> --
>>> Robert Kern
>>>
>>
>> FWIW I'm +1 on the idea. Perhaps because I just don't see many practical
>> downsides to breaking the convention but I regularly see a big issue with there
>> being no way to instantiate an array with a particular value.
>>
>> The one obvious way to do it is use ones and multiply by the value you want. I
>> work with a lot of inexperienced programmers and I see this idiom all the time.
>> It takes a fair amount of numpy knowledge to know that you should do it in two
>> lines by using empty and setting a slice.
>>
>> In [1]: %timeit NaN*ones(10000)
>> 1000 loops, best of 3: 1.74 ms per loop
>>
>> In [2]: %%timeit
>>    ...: x = empty(10000, dtype=float)
>>    ...: x[:] = NaN
>>    ...:
>> 10000 loops, best of 3: 28 us per loop
>>
>> In [3]: 1.74e-3/28e-6
>> Out[3]: 62.142857142857146
>>
>>
>> Even when not in the mythical "tight loop" setting an array to one and then
>> multiplying uses up a lot of cycles - it's nearly 2 orders of magnitude slower
>> than what we know they *should* be doing.
>>
>> I'm agnostic as to whether fill should be modified or new functions provided but
>> I think numpy is currently missing this functionality and that providing it
>> would save a lot of new users from shooting themselves in the foot performance-
>> wise.
>
> Is this a fair summary?
>
> => fill(shape, val), fill_like(arr, val) - new functions, as proposed
> For: readable, seems to fit a pattern often used, presence in
> namespace may clue people into using the 'fill' rather than * val or +
> val
> Con: a very simple alias for a = ones(shape) ; a.fill(val), maybe
> cluttering already full namespace.
>
> => empty(shape).fill(val) - by allowing return value from arr.fill(val)
> For: readable
> Con: breaks guideline not to return anything from in-place operations,
> no presence in namespace means users may not find this pattern.
>
> => no new API
> For : easy maintenance
> Con : harder for users to discover fill pattern, filling a new array
> requires two lines instead of one.
>
> So maybe the decision rests on:
>
> How important is it that users see these function names in the
> namespace in order to discover the pattern "a = ones(shape) ;
> a.fill(val)"?
>
> How important is it to obey guidelines for no-return-from-in-place?
>
> How important is it to avoid expanding the namespace?
>
> How common is this pattern?
>
> On the last, I'd say that the only common use I have for this pattern
> is to fill an array with NaN.

My 2 cts from a user perspective:

- +1 to have such a function. I usually use numpy.ones * scalar
because honestly, spending two lines of code for such a basic
operations seems like a waste. Even if it's slower and potentially
dangerous due to casting rules.
- I think having a noun rather than a verb makes more sense since we
have numpy.ones and numpy.zeros (and I always read "numpy.empty" as
"give me an empty array", not "empty an array").
- I agree the name collision with np.ma.filled is a problem. I have no
better suggestion though at this point.

-=- Olivier


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