[Python-ideas] Re: Allowing assertEqual etc to take multiple items

[Steven D'Aprano]

On Thu, May 28, 2020 at 11:40:52AM +0300, Serhiy Storchaka wrote:

> Why do you think it will be faster? `a == b == c` is the same as `a == b 
> and b == c`, but a tiny bit slower.

That surprises me. I thought that the big advantage of chained 
comparisons is that results are only evaluated once.

a == expensive_expression == c

should be faster than calculating the expensive expression twice:

a == expensive_expression and expensive_expression == c

and even if the expression is just a name lookup, isn't one name lookup 
cheaper than two?

> You always can write
> 
> self.assertTrue(a == b == c)
> 
> But the advantage of two separate assertions is that you know what 
> comparison fails if it fails and get more informative report.

I agree that this is a very good argument for writing separate 
assertions.


-- 
Steven
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[Python-ideas] Re: Allowing assertEqual etc to take multiple items

[Ram Rachum]

On Thu, May 28, 2020 at 11:42 AM Serhiy Storchaka 
wrote:

> 28.05.20 11:02, Ram Rachum пише:
> > I recently submitted this PR
> > , and
> I
> > had to use assertSequenceEqual twice, just because I wanted to compare 3
> > sequences to each other rather than 2.
>
> You have wrote much more text in this message that you would save by not
> typing assertSequenceEqual twice.
>

I see text written in emails as about x10,000 cheaper than text written in
code.


>
> > Wouldn't it be possible to make assertEqual and its entire family
> > (including assertSequenceEqual and the dozen of other methods) accept an
> > arbitrary number of arguments, and compare all of them to each other?
> > That way I could do this:
> >
> >  self.assertSequenceEqual(sliced_tuple_0, sliced_tuple_1,
> sliced_range)
>
> As Rémi said, it is breaking change.
>

I understand. We *could *slowly move towards having these keyword arguments
be keyword-only arguments, with a patient deprecation cycle. This would
probably be a good thing regardless. But I know that's a hard case to make
here.
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[Python-ideas] Re: Allowing assertEqual etc to take multiple items

[Serhiy Storchaka]

28.05.20 11:02, Ram Rachum пише:
I recently submitted this PR 
, and I 
had to use assertSequenceEqual twice, just because I wanted to compare 3 
sequences to each other rather than 2.


You have wrote much more text in this message that you would save by not 
typing assertSequenceEqual twice.


Wouldn't it be possible to make assertEqual and its entire family 
(including assertSequenceEqual and the dozen of other methods) accept an 
arbitrary number of arguments, and compare all of them to each other? 
That way I could do this:


     self.assertSequenceEqual(sliced_tuple_0, sliced_tuple_1, sliced_range)


As Rémi said, it is breaking change.

We could discuss whether they'll do the faster version of a == b == c, 
or a more extensive check between every 2 items individually.


Why do you think it will be faster? `a == b == c` is the same as `a == b 
and b == c`, but a tiny bit slower.


You always can write

self.assertTrue(a == b == c)

But the advantage of two separate assertions is that you know what 
comparison fails if it fails and get more informative report.

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[Python-ideas] Re: Allowing assertEqual etc to take multiple items

[remi . lapeyre]

Hi Ram, this is not possible without breaking a lot of code, 
assertSequenceEqual() already takes a third argument:



Help on function assertSequenceEqual in module unittest.case:

assertSequenceEqual(self, seq1, seq2, msg=None, seq_type=None)
An equality assertion for ordered sequences (like lists and tuples).

For the purposes of this function, a valid ordered sequence type is one
which can be indexed, has a length, and has an equality operator.

Args:
seq1: The first sequence to compare.
seq2: The second sequence to compare.
seq_type: The expected datatype of the sequences, or None if no
datatype should be enforced.
msg: Optional message to use on failure instead of a list of
differences.
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