Minimal test case for Pytest 5 failure with Flask.

test_pytest_failure.py

import flask

def test_pytest_failure():
    """
    This test passes on Pytest 4, but throws an error on Pytest 5.
    """
    app = flask.Flask(__name__)

zsh-console.log

$ pytest test_pytest_failure.py 
============================= test session starts ==============================
platform darwin -- Python 3.7.3, pytest-5.0.0, py-1.8.0, pluggy-0.12.0
rootdir: /private/tmp
collected 1 item                                                               

test_pytest_failure.py F                                                 [100%]

=================================== FAILURES ===================================
_____________________________ test_pytest_failure ______________________________

    def test_pytest_failure():
        """
        This test passes on Pytest 4, but throws an error on Pytest 5.
        """
>       app = flask.Flask(__name__)

test_pytest_failure.py:7: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
/Users/singingwolfboy/.virtualenvs/tmp/lib/python3.7/site-packages/flask/app.py:381: in __init__
    instance_path = self.auto_find_instance_path()
/Users/singingwolfboy/.virtualenvs/tmp/lib/python3.7/site-packages/flask/app.py:678: in auto_find_instance_path
    prefix, package_path = find_package(self.import_name)
/Users/singingwolfboy/.virtualenvs/tmp/lib/python3.7/site-packages/flask/helpers.py:826: in find_package
    loader, root_mod_name):
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

loader = <_pytest.assertion.rewrite.AssertionRewritingHook object at 0x10fb42208>
mod_name = 'test_pytest_failure'

    def _matching_loader_thinks_module_is_package(loader, mod_name):
        """Given the loader that loaded a module and the module this function
        attempts to figure out if the given module is actually a package.
        """
        # If the loader can tell us if something is a package, we can
        # directly ask the loader.
        if hasattr(loader, 'is_package'):
            return loader.is_package(mod_name)
        # importlib's namespace loaders do not have this functionality but
        # all the modules it loads are packages, so we can take advantage of
        # this information.
        elif (loader.__class__.__module__ == '_frozen_importlib' and
              loader.__class__.__name__ == 'NamespaceLoader'):
            return True
        # Otherwise we need to fail with an error that explains what went
        # wrong.
        raise AttributeError(
            ('%s.is_package() method is missing but is required by Flask of '
             'PEP 302 import hooks.  If you do not use import hooks and '
             'you encounter this error please file a bug against Flask.') %
>           loader.__class__.__name__)
E       AttributeError: AssertionRewritingHook.is_package() method is missing but is required by Flask of PEP 302 import hooks.  If you do not use import hooks and you encounter this error please file a bug against Flask.

/Users/singingwolfboy/.virtualenvs/tmp/lib/python3.7/site-packages/flask/helpers.py:789: AttributeError
=============================== warnings summary ===============================
/Users/singingwolfboy/.virtualenvs/tmp/lib/python3.7/site-packages/jinja2/utils.py:485
  /Users/singingwolfboy/.virtualenvs/tmp/lib/python3.7/site-packages/jinja2/utils.py:485: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
    from collections import MutableMapping

/Users/singingwolfboy/.virtualenvs/tmp/lib/python3.7/site-packages/jinja2/runtime.py:318
  /Users/singingwolfboy/.virtualenvs/tmp/lib/python3.7/site-packages/jinja2/runtime.py:318: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
    from collections import Mapping

-- Docs: https://docs.pytest.org/en/latest/warnings.html
===================== 1 failed, 2 warnings in 0.22 seconds =====================