diff --git a/CHANGES.rst b/CHANGES.rst
index 73b5eb98..59193bf7 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -94,6 +94,7 @@ Unreleased
     requires upgrading to Werkzeug 0.15.5. :issue:`3249`
 -   ``send_file`` url quotes the ":" and "/" characters for more
     compatible UTF-8 filename support in some browsers. :issue:`3074`
+-   Fixes for PEP451 import loaders and pytest 5.x.  :issue:`3275`
 
 
 Version 1.0.3
diff --git a/docs/tutorial/tests.rst b/docs/tutorial/tests.rst
index 656f014c..e8b14e4c 100644
--- a/docs/tutorial/tests.rst
+++ b/docs/tutorial/tests.rst
@@ -188,7 +188,7 @@ should be closed.
         with pytest.raises(sqlite3.ProgrammingError) as e:
             db.execute('SELECT 1')
 
-        assert 'closed' in str(e)
+        assert 'closed' in str(e.value)
 
 The ``init-db`` command should call the ``init_db`` function and output
 a message.
diff --git a/examples/tutorial/tests/test_db.py b/examples/tutorial/tests/test_db.py
index 86d2c0e9..31c6c57c 100644
--- a/examples/tutorial/tests/test_db.py
+++ b/examples/tutorial/tests/test_db.py
@@ -13,7 +13,7 @@ def test_get_close_db(app):
     with pytest.raises(sqlite3.ProgrammingError) as e:
         db.execute("SELECT 1")
 
-    assert "closed" in str(e)
+    assert "closed" in str(e.value)
 
 
 def test_init_db_command(runner, monkeypatch):
diff --git a/src/flask/helpers.py b/src/flask/helpers.py
index b121d040..3f401a5b 100644
--- a/src/flask/helpers.py
+++ b/src/flask/helpers.py
@@ -610,7 +610,7 @@ def send_file(
                 "filename": unicodedata.normalize("NFKD", attachment_filename).encode(
                     "ascii", "ignore"
                 ),
-                'filename*': "UTF-8''%s" % url_quote(attachment_filename, safe=b""),
+                "filename*": "UTF-8''%s" % url_quote(attachment_filename, safe=b""),
             }
         else:
             filenames = {"filename": attachment_filename}
@@ -847,19 +847,38 @@ def _matching_loader_thinks_module_is_package(loader, mod_name):
     )
 
 
-def find_package(import_name):
-    """Finds a package and returns the prefix (or None if the package is
-    not installed) as well as the folder that contains the package or
-    module as a tuple.  The package path returned is the module that would
-    have to be added to the pythonpath in order to make it possible to
-    import the module.  The prefix is the path below which a UNIX like
-    folder structure exists (lib, share etc.).
-    """
-    root_mod_name = import_name.split(".")[0]
+def _find_package_path(root_mod_name):
+    """Find the path where the module's root exists in"""
+    if sys.version_info >= (3, 4):
+        import importlib.util
+
+        try:
+            spec = importlib.util.find_spec(root_mod_name)
+            if spec is None:
+                raise ValueError("not found")
+        # ImportError: the machinery told us it does not exist
+        # ValueError:
+        #    - the module name was invalid
+        #    - the module name is __main__
+        #    - *we* raised `ValueError` due to `spec` being `None`
+        except (ImportError, ValueError):
+            pass  # handled below
+        else:
+            # namespace package
+            if spec.origin in {"namespace", None}:
+                return os.path.dirname(next(iter(spec.submodule_search_locations)))
+            # a package (with __init__.py)
+            elif spec.submodule_search_locations:
+                return os.path.dirname(os.path.dirname(spec.origin))
+            # just a normal module
+            else:
+                return os.path.dirname(spec.origin)
+
+    # we were unable to find the `package_path` using PEP 451 loaders
     loader = pkgutil.get_loader(root_mod_name)
-    if loader is None or import_name == "__main__":
+    if loader is None or root_mod_name == "__main__":
         # import name is not found, or interactive/main module
-        package_path = os.getcwd()
+        return os.getcwd()
     else:
         # For .egg, zipimporter does not have get_filename until Python 2.7.
         if hasattr(loader, "get_filename"):
@@ -873,8 +892,8 @@ def find_package(import_name):
             # Google App Engine's HardenedModulesHook
             #
             # Fall back to imports.
-            __import__(import_name)
-            filename = sys.modules[import_name].__file__
+            __import__(root_mod_name)
+            filename = sys.modules[root_mod_name].__file__
         package_path = os.path.abspath(os.path.dirname(filename))
 
         # In case the root module is a package we need to chop of the
@@ -883,6 +902,19 @@ def find_package(import_name):
         if _matching_loader_thinks_module_is_package(loader, root_mod_name):
             package_path = os.path.dirname(package_path)
 
+    return package_path
+
+
+def find_package(import_name):
+    """Finds a package and returns the prefix (or None if the package is
+    not installed) as well as the folder that contains the package or
+    module as a tuple.  The package path returned is the module that would
+    have to be added to the pythonpath in order to make it possible to
+    import the module.  The prefix is the path below which a UNIX like
+    folder structure exists (lib, share etc.).
+    """
+    root_mod_name, _, _ = import_name.partition(".")
+    package_path = _find_package_path(root_mod_name)
     site_parent, site_folder = os.path.split(package_path)
     py_prefix = os.path.abspath(sys.prefix)
     if package_path.startswith(py_prefix):
diff --git a/tests/test_basic.py b/tests/test_basic.py
index b46a7908..8ebdaba4 100644
--- a/tests/test_basic.py
+++ b/tests/test_basic.py
@@ -1220,21 +1220,17 @@ def test_response_type_errors():
 
     with pytest.raises(TypeError) as e:
         c.get("/none")
-
-    assert "returned None" in str(e)
+        assert "returned None" in str(e.value)
 
     with pytest.raises(TypeError) as e:
         c.get("/small_tuple")
-
-    assert "tuple must have the form" in str(e)
+        assert "tuple must have the form" in str(e.value)
 
     pytest.raises(TypeError, c.get, "/large_tuple")
 
     with pytest.raises(TypeError) as e:
         c.get("/bad_type")
-
-    assert "object is not callable" not in str(e)
-    assert "it was a bool" in str(e)
+        assert "it was a bool" in str(e.value)
 
     pytest.raises(TypeError, c.get, "/bad_wsgi")
 
@@ -1636,7 +1632,7 @@ def test_debug_mode_complains_after_first_request(app, client):
         def broken():
             return "Meh"
 
-    assert "A setup function was called" in str(e)
+    assert "A setup function was called" in str(e.value)
 
     app.debug = False
 
@@ -1691,8 +1687,10 @@ def test_routing_redirect_debugging(app, client):
     with client:
         with pytest.raises(AssertionError) as e:
             client.post("/foo", data={})
-        assert "http://localhost/foo/" in str(e)
-        assert ("Make sure to directly send your POST-request to this URL") in str(e)
+        assert "http://localhost/foo/" in str(e.value)
+        assert ("Make sure to directly send your POST-request to this URL") in str(
+            e.value
+        )
 
         rv = client.get("/foo", data={}, follow_redirects=True)
         assert rv.data == b"success"
diff --git a/tests/test_helpers.py b/tests/test_helpers.py
index 3d4b2782..78a56221 100644
--- a/tests/test_helpers.py
+++ b/tests/test_helpers.py
@@ -483,8 +483,8 @@ class TestSendfile(object):
     def test_send_file_object_without_mimetype(self, app, req_ctx):
         with pytest.raises(ValueError) as excinfo:
             flask.send_file(StringIO("LOL"))
-        assert "Unable to infer MIME-type" in str(excinfo)
-        assert "no filename is available" in str(excinfo)
+        assert "Unable to infer MIME-type" in str(excinfo.value)
+        assert "no filename is available" in str(excinfo.value)
 
         flask.send_file(StringIO("LOL"), attachment_filename="filename")