#30546: SimpleLazyObject not so lazy on Python3
               Reporter:  SÅ‚awek     |          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          |
 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
   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
 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
 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>
Django <https://code.djangoproject.com/>
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