#30546: SimpleLazyObject not so lazy on Python3
-------------------------------------+-------------------------------------
Reporter: Sławek | Owner: nobody
Ehlert |
Type: Bug | Status: new
Component: Utilities | Version: 2.2
Severity: Normal | Keywords: SimpleLazyObject
Triage Stage: | lazy evaluation
Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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When running this little script:
{{{
from django.utils.functional import *
def a_func():
raise Exception('naah')
slo = SimpleLazyObject(a_func)
assert not isinstance(slo, type)
}}}
I'd expect the assertion to pass. It works as I'd expect it to work on
Django 1.11 and Python 2.7. However, it does blow up with the {{{naah}}}
exception on both Django 1.11 and 2.2 on Python 3.5 (also on Python 3.6
and 3.7) with the following traceback:
{{{
Traceback (most recent call last):
File "<input>", line 1, in <module>
assert not isinstance(slo, type)
File "... /lib/python3.5/site-packages/django/utils/functional.py", line
238, in inner
self._setup()
File "... /lib/python3.5/site-packages/django/utils/functional.py", line
386, in _setup
self._wrapped = self._setupfunc()
File "<input>", line 2, in a_func
raise Exception('naah')
Exception: naah
}}}
I'm not sure if this is by design, but the behaviour is certainly
inconsistent on Py3 vs. Py2.
I found this problem while trying to run our test suite on Python 3.
Unittest framework does an obvious {{{if isinstance(obj, type) and
issubclass(obj, case.TestCase):}}} check when collecting the test suite
(see
https://github.com/python/cpython/blob/v3.5.7/Lib/unittest/loader.py#L122),
so when we have a module-level {{{SimpleLazyObject}}} (obviously it wraps
a different function than the one I've given here, but also a failing) it
gets unexpectedly evaluated and raises an exception, which prevents us
from even gathering the tests on Python 3.
I'm not really sure why this is happening. After some brief debugging, it
looks like {{{isinstance}}} function on Python 3 is accessing the
{{{__class__}}} attribute (which is wrapped with the
{{{new_method_proxy}}} function - see
https://github.com/django/django/blob/2.2.2/django/utils/functional.py#L348)
causing evaluation of the underlying function, whereas on Python 2.7
{{{isinstance}}} doesn't seem to do that.
--
Ticket URL: <https://code.djangoproject.com/ticket/30546>
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