Re: restriction on sum: intentional bug?

Jon Clements wrote:
On Oct 16, 5:59 pm, Tim Chase <python.l...@xxxxxxxxxxxxxxxxx> wrote:

Stephen Hansen wrote:

Why doesn't duck typing apply to `sum`?

Because it would be so hideously slow and inefficient that it'd be way too
easy a way for people to program something they think should work fine but
really doesn't... alternatively, the function would have to do two
/completely/ different implementations based on what you're passing in, and
that violates duck typing too :)

But that's the issue...supporting strings does NOT involve "two
/completely/ different implementations based on what you're
passing in" but rather just a reduction of starting point
(whether 0 or '') and an object that had an __add__ method.
String meet these requirements. Specifically disqualifying
strings is where you get the two code-paths/implementations.

So I agree with Alan & Peter that this creates an unfortunate
language wart among (as Peter aptly puts it) "consenting adults".
I'd feel similarly if certain classes anomalously prevented
access to internal "private" data. We know that if we go mucking
around like this, it's our own fault if things break or get slow.
Is Python going to prevent me from typing

count = 0
for i in range(1000000):
if i % 1000: count += 1

instead of specifying the step-size? Or even forcing me to
precompute this constant value for `count` because looping is
inefficient in this case?

Even more annoyingly, sum() *knows the right thing to do* to the
degree that it even puts it in the error message...INSTEAD OF
JUST FREAKIN' DOING THE RIGHT THING. Do it inefficiently, or do
it efficiently, but it's NOT AN ERROR. (IMHO ;-)
</soapbox>

-tkc


Does it know the right thing to do though?

For instance, sum(['1', '2', '3']); it's not completely unreasonable
for someone to expect
a result of 6.

It is in Python. '1' + '2' + '3' == '123', and so should sum.

If sum already has two code branches to handle strings, let the string branch do a join. That's my vote, anyway. :)

~Ethan~
.

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