Make error messages more error friendly.

Avoid weird exception chaining to assertion errors.

docs/project/changelog.rst

@@ -56,6 +56,15 @@ They may change at any time.
     If you raise :exc:`~exceptions.ConnectionClosed` or a subclass — rather
     than catch them when websockets raises them — you must change your code.
 
+.. note::
+
+    **Version 10.0 adds a ``msg`` parameter to** ``InvalidURI.__init__`` **.**
+
+    If you raise :exc:`~exceptions.InvalidURI` — rather than catch them when
+    websockets raises them — you must change your code.
+
+Also:
+
 * Added compatibility with Python 3.10.
 
 * Added :func:`~websockets.broadcast` to send a message to many clients.
@@ -150,6 +159,8 @@ They may change at any time.
         from websockets.client import connect
         from websockets.server import serve
 
+Also:
+
 * Added compatibility with Python 3.9.
 
 * Added support for IRIs in addition to URIs.

src/websockets/exceptions.py

@@ -374,11 +374,12 @@ class InvalidURI(WebSocketException):
 
     """
 
-    def __init__(self, uri: str) -> None:
+    def __init__(self, uri: str, msg: str) -> None:
         self.uri = uri
+        self.msg = msg
 
     def __str__(self) -> str:
-        return f"{self.uri} isn't a valid URI"
+        return f"{self.uri} isn't a valid URI: {self.msg}"
 
 
 class PayloadTooBig(WebSocketException):

src/websockets/uri.py

@@ -54,26 +54,27 @@ def parse_uri(uri: str) -> WebSocketURI:
 
     """
     parsed = urllib.parse.urlparse(uri)
-    try:
-        assert parsed.scheme in ["ws", "wss"]
-        assert parsed.params == ""
-        assert parsed.fragment == ""
-        assert parsed.hostname is not None
-    except AssertionError as exc:
-        raise exceptions.InvalidURI(uri) from exc
+    if parsed.scheme not in ["ws", "wss"]:
+        raise exceptions.InvalidURI(uri, "scheme isn't ws or wss")
+    if parsed.hostname is None:
+        raise exceptions.InvalidURI(uri, "hostame isn't provided")
+    if parsed.fragment != "":
+        raise exceptions.InvalidURI(uri, "fragment identifier is meaningless")
 
     secure = parsed.scheme == "wss"
     host = parsed.hostname
     port = parsed.port or (443 if secure else 80)
     resource_name = parsed.path or "/"
+    if parsed.params:
+        resource_name += ";" + parsed.params
     if parsed.query:
         resource_name += "?" + parsed.query
     user_info = None
     if parsed.username is not None:
         # urllib.parse.urlparse accepts URLs with a username but without a
         # password. This doesn't make sense for HTTP Basic Auth credentials.
         if parsed.password is None:
-            raise exceptions.InvalidURI(uri)
+            raise exceptions.InvalidURI(uri, "username provided without password")
         user_info = (parsed.username, parsed.password)
 
     try:

tests/test_exceptions.py

@@ -142,8 +142,8 @@ def test_str(self):
                 "WebSocket connection isn't established yet",
             ),
             (
-                InvalidURI("|"),
-                "| isn't a valid URI",
+                InvalidURI("|", "not at all!"),
+                "| isn't a valid URI: not at all!",
             ),
             (
                 PayloadTooBig("payload length exceeds limit: 2 > 1 bytes"),