Mailing List Archive

On Tue, 20 Jun 2023 at 02:37, Peter Bona via Python-list
<python-list@python.org> wrote:
>
> Hi
>
> I am wondering if there has been any discussion why NoneType is not iterable My feeling is that it should be.
> Sometimes I am using API calls which return None.
> If there is a return value (which is iterable) I am using a for loop to iterate.
>
> Now I am getting 'TypeError: 'NoneType' object is not iterable'.
>
> (Examples are taken from here https://rollbar.com/blog/python-typeerror-nonetype-object-is-not-iterable/)
> Example 1:
> mylist = None
> for x in mylist:
> print(x) <== will raise TypeError: 'NoneType' object is not iterable
> Solution: extra If statement
> if mylist is not None:
> for x in mylist:
> print(x)
>
>
> I think Python should handle this case gracefully: if a code would iterate over None: it should not run any step. but proceed the next statement.
>
> Has this been discussed or proposed?
>

Try this instead:

for x in mylist or ():

Now a None list will skip iteration entirely, allowing you to get the
effect you want :)

ChrisA
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On Mon, Jun 19, 2023 at 12:42?PM Chris Angelico via Python-list <
python-list@python.org> wrote:

> On Tue, 20 Jun 2023 at 02:37, Peter Bona via Python-list
> <python-list@python.org> wrote:
> >
> > Hi
> >
> > I am wondering if there has been any discussion why NoneType is not
> iterable My feeling is that it should be.
> > Sometimes I am using API calls which return None.
> > If there is a return value (which is iterable) I am using a for loop to
> iterate.
> >
> > Now I am getting 'TypeError: 'NoneType' object is not iterable'.
> >
> > (Examples are taken from here
> https://rollbar.com/blog/python-typeerror-nonetype-object-is-not-iterable/
> )
> > Example 1:
> > mylist = None
> > for x in mylist:
> > print(x) <== will raise TypeError: 'NoneType' object is not iterable
> > Solution: extra If statement
> > if mylist is not None:
> > for x in mylist:
> > print(x)
> >
> >
> > I think Python should handle this case gracefully: if a code would
> iterate over None: it should not run any step. but proceed the next
> statement.
> >
> > Has this been discussed or proposed?
> >
>
> Try this instead:
>
> for x in mylist or ():
>
> Now a None list will skip iteration entirely, allowing you to get the
> effect you want :)
>
> ChrisA
> --
> https://mail.python.org/mailman/listinfo/python-list
>
I prefer iteration of None to be an error, as in my usage it usually
indicates a mistake that I'd want to catch
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On 20/06/2023 06.12, Neal Becker via Python-list wrote:
> On Mon, Jun 19, 2023 at 12:42?PM Chris Angelico via Python-list <
> python-list@python.org> wrote:
>
>> On Tue, 20 Jun 2023 at 02:37, Peter Bona via Python-list
>> <python-list@python.org> wrote:
>>>
>>> Hi
>>>
>>> I am wondering if there has been any discussion why NoneType is not
>> iterable My feeling is that it should be.
>>> Sometimes I am using API calls which return None.
>>> If there is a return value (which is iterable) I am using a for loop to
>> iterate.
>>>
>>> Now I am getting 'TypeError: 'NoneType' object is not iterable'.
>>>
>>> (Examples are taken from here
>> https://rollbar.com/blog/python-typeerror-nonetype-object-is-not-iterable/
>> )
>>> Example 1:
>>> mylist = None
>>> for x in mylist:
>>> print(x) <== will raise TypeError: 'NoneType' object is not iterable
>>> Solution: extra If statement
>>> if mylist is not None:
>>> for x in mylist:
>>> print(x)
>>>
>>>
>>> I think Python should handle this case gracefully: if a code would
>> iterate over None: it should not run any step. but proceed the next
>> statement.
>>>
>>> Has this been discussed or proposed?
>>>
>>
>> Try this instead:
>>
>> for x in mylist or ():
>>
>> Now a None list will skip iteration entirely, allowing you to get the
>> effect you want :)
>>
>> ChrisA
>> --
>> https://mail.python.org/mailman/listinfo/python-list
>>
> I prefer iteration of None to be an error, as in my usage it usually
> indicates a mistake that I'd want to catch

Agreed!

A better approach is that the API return (perhaps a tuple of) both
"status" and "return_value", rather than overloading the latter.

That said, apparently the OP use-case is for when there is no interest
in status/catch, eg where a 'nothing' answer is THE answer.

--
Regards,
=dn
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On Monday, June 19, 2023 at 4:11:15?PM UTC-4, Julio Di Egidio wrote:
> On Monday, 19 June 2023 at 21:58:21 UTC+2, dn wrote:
> > On 20/06/2023 06.12, Neal Becker via Python-list wrote:
>
> > > I prefer iteration of None to be an error, as in my usage it usually
> > > indicates a mistake that I'd want to catch
> >
> > Agreed!
> That is a *bad practice* with potentially disastrous consequences,
> as I have explained. You too, like Angelico, can't even read?
>
> [snip]
>
> Julio

Julio,

Most of the regulars in this list/group read the posts using the mailing list and
have declared that they explicitly filter out anything posted on Google Groups.

I've seen it multiple times with your posts. You respond to something and others post parallel to you.
It looks like you're being ignored. However, they simply don't see your posts.

Neither they'll see my response.

Regards,
Igor.
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I would question the wisdom of designing an API that
can return either a sequence or None. If it normally
returns a sequence, and there are no items to return,
it should return an empty sequence.

--
Greg

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Op 20/06/2023 om 2:50 schreef Greg Ewing via Python-list:
> I would question the wisdom of designing an API that
> can return either a sequence or None. If it normally
> returns a sequence, and there are no items to return,
> it should return an empty sequence.
I guess it depends on the reason why there are no items. If it is simply
because there are no items, then yes, I agree it should return an empty
sequence. But if it is because of some kind of error condition, I don't
think it should. But in that case I don't think the API should return
None either: I feel it should raise an exception.

--
"Je ne suis pas d’accord avec ce que vous dites, mais je me battrai jusqu’à
la mort pour que vous ayez le droit de le dire."
-- Attribué à Voltaire
"I disapprove of what you say, but I will defend to the death your right to
say it."
-- Attributed to Voltaire
"Ik ben het niet eens met wat je zegt, maar ik zal je recht om het te zeggen
tot de dood toe verdedigen"
-- Toegeschreven aan Voltaire

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> On 20 Jun 2023, at 01:57, Greg Ewing via Python-list <python-list@python.org> wrote:
>
> I would question the wisdom of designing an API that
> can return either a sequence or None.

I have some APIs that do return None or a list.
The None says that a list is not available and that the caller is
responsible with dealing in a application-domain specific with
with that situation.

In other cases I have API that returns a default list, often [].

It all depends on the design needs.

Barry



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On 20/06/23 7:36 pm, Barry wrote:
> I have some APIs that do return None or a list.
> The None says that a list is not available and that the caller is
> responsible with dealing in a application-domain specific with
> with that situation.

In that case, the caller should probably be checking for
None rather than blindly trying to iterate over the result.

--
Greg
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On Wed, 21 Jun 2023 at 03:46, Igor Berger via Python-list
<python-list@python.org> wrote:
> Most of the regulars in this list/group read the posts using the mailing list and
> have declared that they explicitly filter out anything posted on Google Groups.
>
> I've seen it multiple times with your posts. You respond to something and others post parallel to you.
> It looks like you're being ignored. However, they simply don't see your posts.
>
> Neither they'll see my response.
>

I don't filter out Google Groups. However, the mailing list admins
have banned certain users, and their posts do not make it across the
gateway. So for those particular people, NONE of us see their posts.

If you post on Google Groups and it seems like almost nobody is
reading your posts, check whether you've been consistently violating
the Python Code of Conduct. Maybe the problem isn't with everyone
else.

ChrisA
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On 21Jun2023 03:01, Greg Ewing <greg.ewing@canterbury.ac.nz> wrote:
>On 20/06/23 7:36 pm, Barry wrote:
>>I have some APIs that do return None or a list.
>>The None says that a list is not available and that the caller is
>>responsible with dealing in a application-domain specific with
>>with that situation.
>
>In that case, the caller should probably be checking for
>None rather than blindly trying to iterate over the result.

I wasted some time the other evening on an API which returned a string
or None. My own API, and the pain it caused tells me that that API
design choice isn't good (it's an automatic attribute based on a tag,
returning None if the tag isn't there). My experience so far is that it
_looks_ handy so that you can safely say "foo.bar" all the time, but as
soon a you do something with the value you're in for a world of
None-checking.

I'm rethinking that choice right now. Just the other day I removed a
setting in a related class which provided an automatic default value
because, again, while handy for careless use it caused hard to debug
problems because the default would flow out the call chain until it was
unsuitable, making the cause hard to trace.

And of course I'm very -1 on None acquiring iteration or other features.
Fail early, fail often!

Cheers,
Cameron Simpson <cs@cskk.id.au>
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On Wed, 21 Jun 2023 at 09:59, Cameron Simpson via Python-list
<python-list@python.org> wrote:

> I wasted some time the other evening on an API which returned a string
> or None. My own API, and the pain it caused tells me that that API
> design choice isn't good (it's an automatic attribute based on a tag,
> returning None if the tag isn't there). My experience so far is that it
> _looks_ handy so that you can safely say "foo.bar" all the time, but as
> soon a you do something with the value you're in for a world of
> None-checking.
>
https://peps.python.org/pep-0505/

ChrisA
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On 21Jun2023 10:09, Chris Angelico <rosuav@gmail.com> wrote:
>On Wed, 21 Jun 2023 at 09:59, Cameron Simpson via Python-list
><python-list@python.org> wrote:
>> I wasted some time the other evening on an API which returned a
>> string
>> or None. My own API, and the pain it caused tells me that that API
>> design choice isn't good (it's an automatic attribute based on a tag,
>> returning None if the tag isn't there). My experience so far is that it
>> _looks_ handy so that you can safely say "foo.bar" all the time, but as
>> soon a you do something with the value you're in for a world of
>> None-checking.
>>
>https://peps.python.org/pep-0505/


Hmm. Thanks. - Cameron Simpson <cs@cskk.id.au>
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