Simplify untangling of import cycles

Three imports at the bottom, related to type annotations, is a lesser
evil compared to dozens of module-prefixed identifiers, a departure
from the coding style of this library.

Refs #989.

Author: aaugustin

src/websockets/exceptions.py

@@ -33,7 +33,6 @@
 import typing
 import warnings
 
-from . import frames, http11
 from .imports import lazy_import
 
 
@@ -376,3 +375,6 @@ class InvalidState(WebSocketException, AssertionError):
             "WebSocketProtocolError": ".legacy.exceptions",
         },
     )
+
+# At the bottom to break import cycles created by type annotations.
+from . import frames, http11  # noqa: E402

src/websockets/extensions/base.py

@@ -2,7 +2,7 @@
 
 from typing import Sequence
 
-from .. import frames
+from ..frames import Frame
 from ..typing import ExtensionName, ExtensionParameter
 
 
@@ -18,12 +18,7 @@ class Extension:
     name: ExtensionName
     """Extension identifier."""
 
-    def decode(
-        self,
-        frame: frames.Frame,
-        *,
-        max_size: int | None = None,
-    ) -> frames.Frame:
+    def decode(self, frame: Frame, *, max_size: int | None = None) -> Frame:
         """
         Decode an incoming frame.
 
@@ -40,7 +35,7 @@ def decode(
         """
         raise NotImplementedError
 
-    def encode(self, frame: frames.Frame) -> frames.Frame:
+    def encode(self, frame: Frame) -> Frame:
         """
         Encode an outgoing frame.
 

src/websockets/extensions/permessage_deflate.py

@@ -4,7 +4,15 @@
 import zlib
 from typing import Any, Sequence
 
-from .. import exceptions, frames
+from .. import frames
+from ..exceptions import (
+    DuplicateParameter,
+    InvalidParameterName,
+    InvalidParameterValue,
+    NegotiationError,
+    PayloadTooBig,
+    ProtocolError,
+)
 from ..typing import ExtensionName, ExtensionParameter
 from .base import ClientExtensionFactory, Extension, ServerExtensionFactory
 
@@ -129,9 +137,9 @@ def decode(
         try:
             data = self.decoder.decompress(data, max_length)
         except zlib.error as exc:
-            raise exceptions.ProtocolError("decompression failed") from exc
+            raise ProtocolError("decompression failed") from exc
         if self.decoder.unconsumed_tail:
-            raise exceptions.PayloadTooBig(f"over size limit (? > {max_size} bytes)")
+            raise PayloadTooBig(f"over size limit (? > {max_size} bytes)")
 
         # Allow garbage collection of the decoder if it won't be reused.
         if frame.fin and self.remote_no_context_takeover:
@@ -215,40 +223,40 @@ def _extract_parameters(
     for name, value in params:
         if name == "server_no_context_takeover":
             if server_no_context_takeover:
-                raise exceptions.DuplicateParameter(name)
+                raise DuplicateParameter(name)
             if value is None:
                 server_no_context_takeover = True
             else:
-                raise exceptions.InvalidParameterValue(name, value)
+                raise InvalidParameterValue(name, value)
 
         elif name == "client_no_context_takeover":
             if client_no_context_takeover:
-                raise exceptions.DuplicateParameter(name)
+                raise DuplicateParameter(name)
             if value is None:
                 client_no_context_takeover = True
             else:
-                raise exceptions.InvalidParameterValue(name, value)
+                raise InvalidParameterValue(name, value)
 
         elif name == "server_max_window_bits":
             if server_max_window_bits is not None:
-                raise exceptions.DuplicateParameter(name)
+                raise DuplicateParameter(name)
             if value in _MAX_WINDOW_BITS_VALUES:
                 server_max_window_bits = int(value)
             else:
-                raise exceptions.InvalidParameterValue(name, value)
+                raise InvalidParameterValue(name, value)
 
         elif name == "client_max_window_bits":
             if client_max_window_bits is not None:
-                raise exceptions.DuplicateParameter(name)
+                raise DuplicateParameter(name)
             if is_server and value is None:  # only in handshake requests
                 client_max_window_bits = True
             elif value in _MAX_WINDOW_BITS_VALUES:
                 client_max_window_bits = int(value)
             else:
-                raise exceptions.InvalidParameterValue(name, value)
+                raise InvalidParameterValue(name, value)
 
         else:
-            raise exceptions.InvalidParameterName(name)
+            raise InvalidParameterName(name)
 
     return (
         server_no_context_takeover,
@@ -340,7 +348,7 @@ def process_response_params(
 
         """
         if any(other.name == self.name for other in accepted_extensions):
-            raise exceptions.NegotiationError(f"received duplicate {self.name}")
+            raise NegotiationError(f"received duplicate {self.name}")
 
         # Request parameters are available in instance variables.
 
@@ -366,7 +374,7 @@ def process_response_params(
 
         if self.server_no_context_takeover:
             if not server_no_context_takeover:
-                raise exceptions.NegotiationError("expected server_no_context_takeover")
+                raise NegotiationError("expected server_no_context_takeover")
 
         # client_no_context_takeover
         #
@@ -396,9 +404,9 @@ def process_response_params(
 
         else:
             if server_max_window_bits is None:
-                raise exceptions.NegotiationError("expected server_max_window_bits")
+                raise NegotiationError("expected server_max_window_bits")
             elif server_max_window_bits > self.server_max_window_bits:
-                raise exceptions.NegotiationError("unsupported server_max_window_bits")
+                raise NegotiationError("unsupported server_max_window_bits")
 
         # client_max_window_bits
 
@@ -414,7 +422,7 @@ def process_response_params(
 
         if self.client_max_window_bits is None:
             if client_max_window_bits is not None:
-                raise exceptions.NegotiationError("unexpected client_max_window_bits")
+                raise NegotiationError("unexpected client_max_window_bits")
 
         elif self.client_max_window_bits is True:
             pass
@@ -423,7 +431,7 @@ def process_response_params(
             if client_max_window_bits is None:
                 client_max_window_bits = self.client_max_window_bits
             elif client_max_window_bits > self.client_max_window_bits:
-                raise exceptions.NegotiationError("unsupported client_max_window_bits")
+                raise NegotiationError("unsupported client_max_window_bits")
 
         return PerMessageDeflate(
             server_no_context_takeover,  # remote_no_context_takeover
@@ -534,7 +542,7 @@ def process_request_params(
 
         """
         if any(other.name == self.name for other in accepted_extensions):
-            raise exceptions.NegotiationError(f"skipped duplicate {self.name}")
+            raise NegotiationError(f"skipped duplicate {self.name}")
 
         # Load request parameters in local variables.
         (
@@ -613,7 +621,7 @@ def process_request_params(
         else:
             if client_max_window_bits is None:
                 if self.require_client_max_window_bits:
-                    raise exceptions.NegotiationError("required client_max_window_bits")
+                    raise NegotiationError("required client_max_window_bits")
             elif client_max_window_bits is True:
                 client_max_window_bits = self.client_max_window_bits
             elif self.client_max_window_bits < client_max_window_bits:

src/websockets/frames.py

@@ -8,7 +8,7 @@
 import struct
 from typing import Callable, Generator, Sequence
 
-from . import exceptions, extensions
+from .exceptions import PayloadTooBig, ProtocolError
 
 
 try:
@@ -239,10 +239,10 @@ def parse(
         try:
             opcode = Opcode(head1 & 0b00001111)
         except ValueError as exc:
-            raise exceptions.ProtocolError("invalid opcode") from exc
+            raise ProtocolError("invalid opcode") from exc
 
         if (True if head2 & 0b10000000 else False) != mask:
-            raise exceptions.ProtocolError("incorrect masking")
+            raise ProtocolError("incorrect masking")
 
         length = head2 & 0b01111111
         if length == 126:
@@ -252,9 +252,7 @@ def parse(
             data = yield from read_exact(8)
             (length,) = struct.unpack("!Q", data)
         if max_size is not None and length > max_size:
-            raise exceptions.PayloadTooBig(
-                f"over size limit ({length} > {max_size} bytes)"
-            )
+            raise PayloadTooBig(f"over size limit ({length} > {max_size} bytes)")
         if mask:
             mask_bytes = yield from read_exact(4)
 
@@ -342,13 +340,13 @@ def check(self) -> None:
 
         """
         if self.rsv1 or self.rsv2 or self.rsv3:
-            raise exceptions.ProtocolError("reserved bits must be 0")
+            raise ProtocolError("reserved bits must be 0")
 
         if self.opcode in CTRL_OPCODES:
             if len(self.data) > 125:
-                raise exceptions.ProtocolError("control frame too long")
+                raise ProtocolError("control frame too long")
             if not self.fin:
-                raise exceptions.ProtocolError("fragmented control frame")
+                raise ProtocolError("fragmented control frame")
 
 
 @dataclasses.dataclass
@@ -405,7 +403,7 @@ def parse(cls, data: bytes) -> Close:
         elif len(data) == 0:
             return cls(CloseCode.NO_STATUS_RCVD, "")
         else:
-            raise exceptions.ProtocolError("close frame too short")
+            raise ProtocolError("close frame too short")
 
     def serialize(self) -> bytes:
         """
@@ -424,4 +422,8 @@ def check(self) -> None:
 
         """
         if not (self.code in EXTERNAL_CLOSE_CODES or 3000 <= self.code < 5000):
-            raise exceptions.ProtocolError("invalid status code")
+            raise ProtocolError("invalid status code")
+
+
+# At the bottom to break import cycles created by type annotations.
+from . import extensions  # noqa: E402

src/websockets/headers.py

@@ -6,7 +6,7 @@
 import re
 from typing import Callable, Sequence, TypeVar, cast
 
-from . import exceptions
+from .exceptions import InvalidHeaderFormat, InvalidHeaderValue
 from .typing import (
     ConnectionOption,
     ExtensionHeader,
@@ -108,7 +108,7 @@ def parse_token(header: str, pos: int, header_name: str) -> tuple[str, int]:
     """
     match = _token_re.match(header, pos)
     if match is None:
-        raise exceptions.InvalidHeaderFormat(header_name, "expected token", header, pos)
+        raise InvalidHeaderFormat(header_name, "expected token", header, pos)
     return match.group(), match.end()
 
 
@@ -132,9 +132,7 @@ def parse_quoted_string(header: str, pos: int, header_name: str) -> tuple[str, i
     """
     match = _quoted_string_re.match(header, pos)
     if match is None:
-        raise exceptions.InvalidHeaderFormat(
-            header_name, "expected quoted string", header, pos
-        )
+        raise InvalidHeaderFormat(header_name, "expected quoted string", header, pos)
     return _unquote_re.sub(r"\1", match.group()[1:-1]), match.end()
 
 
@@ -206,9 +204,7 @@ def parse_list(
         if peek_ahead(header, pos) == ",":
             pos = parse_OWS(header, pos + 1)
         else:
-            raise exceptions.InvalidHeaderFormat(
-                header_name, "expected comma", header, pos
-            )
+            raise InvalidHeaderFormat(header_name, "expected comma", header, pos)
 
         # Remove extra delimiters before the next item.
         while peek_ahead(header, pos) == ",":
@@ -276,9 +272,7 @@ def parse_upgrade_protocol(
     """
     match = _protocol_re.match(header, pos)
     if match is None:
-        raise exceptions.InvalidHeaderFormat(
-            header_name, "expected protocol", header, pos
-        )
+        raise InvalidHeaderFormat(header_name, "expected protocol", header, pos)
     return cast(UpgradeProtocol, match.group()), match.end()
 
 
@@ -324,7 +318,7 @@ def parse_extension_item_param(
             # the value after quoted-string unescaping MUST conform to
             # the 'token' ABNF.
             if _token_re.fullmatch(value) is None:
-                raise exceptions.InvalidHeaderFormat(
+                raise InvalidHeaderFormat(
                     header_name, "invalid quoted header content", header, pos_before
                 )
         else:
@@ -510,9 +504,7 @@ def parse_token68(header: str, pos: int, header_name: str) -> tuple[str, int]:
     """
     match = _token68_re.match(header, pos)
     if match is None:
-        raise exceptions.InvalidHeaderFormat(
-            header_name, "expected token68", header, pos
-        )
+        raise InvalidHeaderFormat(header_name, "expected token68", header, pos)
     return match.group(), match.end()
 
 
@@ -522,7 +514,7 @@ def parse_end(header: str, pos: int, header_name: str) -> None:
 
     """
     if pos < len(header):
-        raise exceptions.InvalidHeaderFormat(header_name, "trailing data", header, pos)
+        raise InvalidHeaderFormat(header_name, "trailing data", header, pos)
 
 
 def parse_authorization_basic(header: str) -> tuple[str, str]:
@@ -543,12 +535,12 @@ def parse_authorization_basic(header: str) -> tuple[str, str]:
     # https://datatracker.ietf.org/doc/html/rfc7617#section-2
     scheme, pos = parse_token(header, 0, "Authorization")
     if scheme.lower() != "basic":
-        raise exceptions.InvalidHeaderValue(
+        raise InvalidHeaderValue(
             "Authorization",
             f"unsupported scheme: {scheme}",
         )
     if peek_ahead(header, pos) != " ":
-        raise exceptions.InvalidHeaderFormat(
+        raise InvalidHeaderFormat(
             "Authorization", "expected space after scheme", header, pos
         )
     pos += 1
@@ -558,14 +550,14 @@ def parse_authorization_basic(header: str) -> tuple[str, str]:
     try:
         user_pass = base64.b64decode(basic_credentials.encode()).decode()
     except binascii.Error:
-        raise exceptions.InvalidHeaderValue(
+        raise InvalidHeaderValue(
             "Authorization",
             "expected base64-encoded credentials",
         ) from None
     try:
         username, password = user_pass.split(":", 1)
     except ValueError:
-        raise exceptions.InvalidHeaderValue(
+        raise InvalidHeaderValue(
             "Authorization",
             "expected username:password credentials",
         ) from None