Give Port.protocol a default value, as per review.

scenario/state.py

@@ -865,22 +865,20 @@ def handle_path(self):
 class Port:
     """Represents a port on the charm host."""
 
-    protocol: _RawPortProtocolLiteral
+    protocol: _RawPortProtocolLiteral = "tcp"
     port: Optional[int] = None
     """The port to open. Required for TCP and UDP; not allowed for ICMP."""
 
     # NOTE: This can be replaced with dataclass(kw_only=True) in Python 3.10+
     def __init__(
         self,
         *,
-        protocol: _RawPortProtocolLiteral,
+        protocol: _RawPortProtocolLiteral = "tcp",
         port: Optional[int] = None,
     ):
         object.__setattr__(self, "protocol", protocol)
         object.__setattr__(self, "port", port)
 
-    def __post_init__(self):
-        port = self.port
         is_icmp = self.protocol == "icmp"
         if port:
             if is_icmp:

tests/test_e2e/test_ports.py

@@ -2,7 +2,7 @@
 from ops import CharmBase, Framework, StartEvent, StopEvent
 
 from scenario import Context, State
-from scenario.state import Port
+from scenario.state import Port, StateValidationError
 
 
 class MyCharm(CharmBase):
@@ -37,3 +37,25 @@ def test_open_port(ctx):
 def test_close_port(ctx):
     out = ctx.run(ctx.on.stop(), State(opened_ports=[Port(protocol="tcp", port=42)]))
     assert not out.opened_ports
+
+
+def test_port_no_arguments():
+    with pytest.raises(StateValidationError):
+        Port()
+
+
+def test_port_default_protocol():
+    port = Port(port=42)
+    assert port.protocol == "tcp"
+    assert port.port == 42
+
+
+def test_port_port():
+    with pytest.raises(StateValidationError):
+        Port(protocol="icmp", port=42)
+    with pytest.raises(StateValidationError):
+        Port(protocol="tcp", port=0)
+    with pytest.raises(StateValidationError):
+        Port(protocol="udp", port=65536)
+    with pytest.raises(StateValidationError):
+        Port(protocol="tcp")