#31961: forms.ChoiceField calls choices callback multiple times.
---------------------------+--------------------------------------
Reporter: Roy Smith | Owner: nobody
Type: Bug | Status: new
Component: Forms | Version: 2.2
Severity: Normal | Resolution:
Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
---------------------------+--------------------------------------
Description changed by Roy Smith:
Old description:
> I'm running:
>
> Python 3.7
> Django 2.2
> Debian 4.9
>
> If I configure a forms ChoiceField with a callback function for choices,
> it gets called twice each time I render the form:
>
> {{{
> from unittest import TestCase
> from django import forms
>
> def callback():
> print("callback")
> return [('foo', 'bar')]
>
> class MyForm(forms.Form):
> f = forms.ChoiceField(choices=callback)
>
> class FormTest(TestCase):
> MyForm().as_p()
> }}}
>
> prints:
>
> {{{
> ./manage.py test spi.test_f
> callback
> callback
> System check identified no issues (0 silenced).
>
> ----------------------------------------------------------------------
> Ran 0 tests in 0.000s
>
> OK
> }}}
>
> This is the same sort of problem described in #26665 and/or #11390. It's
> an obvious performance issue, but more than that, if can beak behavior if
> the callback is non-idempotent.
>
> I discovered this because I had patched my callback using unttest.mock to
> return a sequence of return values and got hard-to-diagnose test
> failures. Specifically, my test failed because it raised StopIteration
> when it ran out of values to return.
New description:
I'm running:
Python 3.7
Django 2.2
Debian 4.9
If I configure a forms ChoiceField with a callback function for choices,
it gets called twice each time I render the form:
{{{
from unittest import TestCase
from django import forms
def callback():
print("callback")
return [('foo', 'bar')]
class MyForm(forms.Form):
f = forms.ChoiceField(choices=callback)
class FormTest(TestCase):
MyForm().as_p()
}}}
prints:
{{{
./manage.py test spi.test_f
callback
callback
System check identified no issues (0 silenced).
----------------------------------------------------------------------
Ran 0 tests in 0.000s
OK
}}}
This is the same sort of problem described in #26665 and/or #11390. It's
an obvious performance issue, but more than that, it can beak behavior if
the callback is non-idempotent.
I discovered this because I had patched my callback using unttest.mock to
return a sequence of return values and got hard-to-diagnose test failures.
Specifically, my test failed because it raised StopIteration when it ran
out of values to return.
--
--
Ticket URL: <https://code.djangoproject.com/ticket/31961#comment:1>
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