On Mon, May 4, 2020 at 2:33 AM Steven D'Aprano <st...@pearwood.info> wrote:
> On Sat, May 02, 2020 at 07:43:44PM +0200, Alex Hall wrote:
> > On Sat, May 2, 2020 at 6:09 PM Steven D'Aprano <st...@pearwood.info>
> wrote:
> >
> > > On Sat, May 02, 2020 at 04:58:43PM +0200, Alex Hall wrote:
> > >
> > > > I didn't think carefully about this implementation and thought that
> there
> > > > was only a performance cost in the error case. That's obviously not
> true
> > > -
> > > > there's an `if` statement executed in Python for every item in every
> > > > iterable.
> > >
> > > Sorry, this does not demonstrate that the performance cost is
> > > significant.
> > >
> > > This adds one "if" per loop, terminating on (one more than) the
> shortest
> > > input. So O(N) on the length of the input. That's usually considered
> > > reasonable, provided the per item cost is low.
> > >
> > > The test in the "if" is technically O(N) on the number of input
> > > iterators, but since that's usually two, and rarely more than a
> handful,
> > > it's close enough to a fixed cost.
> > >
> > > On my old and slow PC `sentinel in combo` is quite fast:
> > >
> >
> > `sentinel in combo` is problematic if some values have overridden
> `__eq__`.
> > I referred to this problem in a previous email to you, saying that people
> > had copied this buggy implementation from SO and that it still hadn't
> been
> > fixed after being pointed out. The fact that you missed this helps to
> prove
> > my point. Getting this right is hard.
>
> I didn't miss it, I ignored it as YAGNI.
>
> Seriously, if some object defines a weird `__eq__` then half the
> standard library, including builtins, stops working "correctly". See for
> example the behaviour of float NANs in lists.
>
> My care factor for this is negligible, until such time that it is proven
> to be an issue for real objects in real code. Until then, YAGNI.
>
Here is an example:
```
import numpy as np
from itertools import zip_longest
def zip_equal(*iterables):
sentinel = object()
for combo in zip_longest(*iterables, fillvalue=sentinel):
if sentinel in combo:
raise ValueError('Iterables have different lengths')
yield combo
arr = np.arange(8).reshape((2, 2, 2))
print(arr)
print(list(zip(*arr)))
print(list(zip_equal(*arr)))
```
The output:
```
[[[0 1]
[2 3]]
[[4 5]
[6 7]]]
[(array([0, 1]), array([4, 5])), (array([2, 3]), array([6, 7]))]
Traceback (most recent call last):
File
"/home/alex/.config/JetBrains/PyCharm2020.1/scratches/scratch_666.py", line
15, in <module>
print(list(zip_equal(*arr)))
File
"/home/alex/.config/JetBrains/PyCharm2020.1/scratches/scratch_666.py", line
8, in zip_equal
if sentinel in combo:
ValueError: The truth value of an array with more than one element is
ambiguous. Use a.any() or a.all()
```
I know for a fact that this would confuse people badly because I've seen
multiple people who know what this error message generally refers to
incorrectly identify where exactly it's coming from in a similar case:
https://stackoverflow.com/questions/60780328/python-valueerror-the-truth-value-of-an-array-with-more-than-one-element-is-amb/60780361
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