Django 2.0 compatibility: User.is_authenticated is now a property

[sciyoshi]

Fixes #397 - tracing with Django 2.0 seems to work fine with this small change.

[palazzem]

@sciyoshi thank you very much with this PR! Indeed we're looking on it so that we can review/merge for the next release! Will keep you updated!

[palazzem]

Here I think we have a couple of options:

• use a kind of wrapper in a compat.py module inside contrib/django that retrieves properly the field:

def is_authenticated(user):
    if callable(user.is_authenticated):
        return user.is_authenticated()
    return user.is_authenticated

• ask forgiveness and not the permission (that is idiomatic for Python and is what you're doing), but in this case we hit the exception most of the time (so it's not an exception anymore). Any newer version of Django (>=2.0) will have only the boolean and so we'll always hit the except block, introducing the overhead to handle the exception for any request.

Even if I really like your approach, would be great making user.is_authenticated the default case and not the exception since it will be future-proof.

Does it make sense? What do you think?

[sciyoshi]

Yea, the exception probably isn't great for performance. I've updated the PR with a version check and a wrapper in compat.py.

[palazzem]

Thanks @sciyoshi for the change! All good to me, I've just added Django 2.0 to our test matrix and double checked that without your change the test suite fails (and it does).

Let's wait for the CI and see if I need to tweak something else otherwise this will be part of the next release.

Thanks again!