Re: shortest way to recover from a QuerySet "object not found"

[Shawn Milochik]

If the question is answered to your satisfaction, why don't we drop the thread 
now and avoid the personal stuff? 

There is nothing to be gained by arguing with a respected core Django 
developer, and nothing the rest of us will learn from it.

Thanks,
Shawn


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Re: shortest way to recover from a QuerySet "object not found"

[Phlip]

> The only condescension I've seen in this thread is from you. And, to
> be fair, if I wanted to be condescending I'd have simply pointed you
> at Tony Hoare's explanation of null values

That's why I said NullObject, in the first post.

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Re: shortest way to recover from a QuerySet "object not found"

[James Bennett]

On Thu, Mar 4, 2010 at 1:43 PM, Phlip  wrote:
> And again the condescension. As a programmer, I should be able to
> easily chose between statements that throw and ones that efficiently
> deal with branching conditions. A record-not-found is not a crisis, it
> is just a branching condition. 5 excess lines of code do not make my
> program more robust.

The only condescension I've seen in this thread is from you. And, to
be fair, if I wanted to be condescending I'd have simply pointed you
at Tony Hoare's explanation of null values (which he originally
introduced into ALGOL, way back at the dawn of time) as a
"billion-dollar mistake" ;)

In general, though, Django optimizes for the most common cases.
Experience has shown that, in the case of a record not being found,
there are two things people most often want to do:

1. Bail out and return an HTTP 404 response.
2. Create a record which matched the specified conditions.

As a result, Django provides one-line shortcuts which implement both
of those use cases (get_object_or_404() and get_or_create()). Creating
a null object to pass around, meanwhile, is less common and -- as
mentioned -- is more of an anti-pattern to be discouraged. As for why
get() raises an exception, well, it *is* an exceptional case; usually,
when you're that specific in asking for an object, you expect it to be
there.

> Oh, look! "123"[:7] ! The string doesn't have 7 characters! Gee,
> shouldn't that throw an exception?

See my comments above about condescension.


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Re: shortest way to recover from a QuerySet "object not found"

[Shawn Milochik]

Just create your own Manager and override the default (named 'objects') in your 
models. Have 'get' behave any way you like.

http://docs.djangoproject.com/en/1.1/topics/db/managers/

Shawn

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Re: shortest way to recover from a QuerySet "object not found"

[Phlip]

> Just create your own Manager and override the default (named 'objects') in 
> your models. Have 'get' behave any way you like.
>
> http://docs.djangoproject.com/en/1.1/topics/db/managers/
>
> Shawn

Ding! 
http://docs.djangoproject.com/en/1.1/topics/db/managers/#adding-extra-manager-methods

luv that Open Closed Principle, huh?

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Re: shortest way to recover from a QuerySet "object not found"

[Phlip]

> Most likely, yes. And those people, believe it or not, designed
> Django's APIs based on their experiences. This is why there are
> shortcuts available, like the get_or_create() method (which fetches an
> object or, if none matches, creates a valid one, populated with
> default values you supply, for you), and why these sorts of methods
> are covered in Django's documentation.

I want a NullObject - not "create", which (correctly) implies a save()

> In general, though, it appears you're trying to apply an anti-pattern:
> silencing an exception which told you that something went wrong, and
> then passing a "null" object into other code to try to compensate.
> This is an approach more likely to create bugs than to solve them, and
> so if implementing it is a bit harder than you'd like, perhaps that's
> why.

And again the condescension. As a programmer, I should be able to
easily chose between statements that throw and ones that efficiently
deal with branching conditions. A record-not-found is not a crisis, it
is just a branching condition. 5 excess lines of code do not make my
program more robust.

Oh, look! "123"[:7] ! The string doesn't have 7 characters! Gee,
shouldn't that throw an exception?

--
  Phlip

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Re: shortest way to recover from a QuerySet "object not found"

[James Bennett]

On Thu, Mar 4, 2010 at 12:59 PM, Phlip  wrote:
> Doesn't anyone in Django-land have experience with the platforms that
> make this problem incredibly easy?

Most likely, yes. And those people, believe it or not, designed
Django's APIs based on their experiences. This is why there are
shortcuts available, like the get_or_create() method (which fetches an
object or, if none matches, creates a valid one, populated with
default values you supply, for you), and why these sorts of methods
are covered in Django's documentation.

In general, though, it appears you're trying to apply an anti-pattern:
silencing an exception which told you that something went wrong, and
then passing a "null" object into other code to try to compensate.
This is an approach more likely to create bugs than to solve them, and
so if implementing it is a bit harder than you'd like, perhaps that's
why.


-- 
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Re: shortest way to recover from a QuerySet "object not found"

[Phlip]

> from django.core.exceptions import ObjectDoesNotExist

txbut... >sigh< I was hoping to head that off - Python's condescending
attitude is in fact the core of the problem.

Even if you wrap all your try: except: up in a method, so it's at
least DRY, is you must consign that method to use .get().

So to squeeze my attempt into one line, we get foo =
(list(Foo.objects.filter(name='bar')) + [Foo()])[0], which is way too
much typing.

Doesn't anyone in Django-land have experience with the platforms that
make this problem incredibly easy?

--
  Phlip

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Re: shortest way to recover from a QuerySet "object not found"

[rebus_]

On 4 March 2010 18:02, Phlip  wrote:
> Djangoids:
>
> Consider this line:
>
>   foo = Foo.object.get(name='bar')
>
> If foo is not found, I want it to contain a NullObject, such as an
> empty Foo(). In the parlance, that could be like this:
>
>   foo = Foo.object.get(name='bar', _default=Foo())
>
> I naturally don't expect (v 1.1.1) of Django to support my magic
> _default keyword.
>
> What's the absolute shortest stretch of code which pops a NullObject
> into my foo if the record 'bar' is not found?
>
> --
>  Phlip
>  http://c2.com/cgi/wiki?ZeekLand
>
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>
>

I'd say:

from django.core.exceptions import ObjectDoesNotExist
try:
  foo = Foo.object.get(name='bar')
except ObjectDoesNotExist:
  foo = bar()

but then again, there are all kinds of magic out there. :)

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shortest way to recover from a QuerySet "object not found"

[Phlip]

Djangoids:

Consider this line:

   foo = Foo.object.get(name='bar')

If foo is not found, I want it to contain a NullObject, such as an
empty Foo(). In the parlance, that could be like this:

   foo = Foo.object.get(name='bar', _default=Foo())

I naturally don't expect (v 1.1.1) of Django to support my magic
_default keyword.

What's the absolute shortest stretch of code which pops a NullObject
into my foo if the record 'bar' is not found?

--
  Phlip
  http://c2.com/cgi/wiki?ZeekLand

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