Fix 0 port value being overriden by default

By explicitly comparing port value with None,
instead of using its bool() value.

CHANGES.rst

@@ -28,9 +28,12 @@ Released on May 2nd 2018
     a blueprint prefix and route. (`#2748`_)
 -   Fix error with ``flask routes`` command when there are no routes.
     (`#2751`_)
+-   Fix issue with port set to 0 being overriden by the default value.
+    (`#2928`_)
 
 .. _#2748: https://github.com/pallets/flask/pull/2748
 .. _#2751: https://github.com/pallets/flask/issues/2751
+.. _#2928: https://github.com/pallets/flask/pull/2928
 
 
 Version 1.0.1

flask/app.py

@@ -929,7 +929,7 @@ class Flask(_PackageBoundObject):
             sn_host, _, sn_port = server_name.partition(':')
 
         host = host or sn_host or _host
-        port = int(port or sn_port or _port)
+        port = int(next((p for p in (port, sn_port) if p is not None), _port))
 
         options.setdefault('use_reloader', self.debug)
         options.setdefault('use_debugger', self.debug)

tests/test_basic.py

@@ -1921,6 +1921,7 @@ def test_run_server_port(monkeypatch, app):
         ('localhost', None, 'localhost', 8080),
         (None, 80, 'pocoo.org', 80),
         ('localhost', 80, 'localhost', 80),
+        ('localhost', 0, 'localhost', 0),
 ))
 def test_run_from_config(monkeypatch, host, port, expect_host, expect_port, app):
     def run_simple_mock(hostname, port, *args, **kwargs):