Fix for metaclass errors in Python 3.6

Author: bryanforbes

asyncpg/connection.py

@@ -103,14 +103,22 @@ def __call__(
         ...
 
 
-class ConnectionMeta(type):
+class _ConnectionMeta(type):
 
     def __instancecheck__(cls, instance: typing.Any) -> bool:
         mro = type(instance).__mro__
         return Connection in mro or _ConnectionProxy in mro
 
 
-class Connection(typing.Generic[_Record], metaclass=ConnectionMeta):
+if sys.version_info >= (3, 7):
+    ConnectionMeta = _ConnectionMeta
+else:
+    # see: https://github.com/python/typing/issues/449
+    class ConnectionMeta(_ConnectionMeta, typing.GenericMeta):
+        ...
+
+
+class Connection(typing.Generic[_Record], metaclass=ConnectionMeta):  # type: ignore[misc]  # noqa: E501
     """A representation of a database session.
 
     Connections are created by calling :func:`~asyncpg.connection.connect`.
@@ -2375,6 +2383,11 @@ async def _do_execute(
         return result, stmt
 
 
+if sys.version_info < (3, 7):
+    # see: https://bugs.python.org/issue41451
+    del Connection.__slots__
+
+
 @typing.overload
 async def connect(dsn: typing.Optional[str] = ..., *,
                   host: typing.Optional[connect_utils.HostType] = ...,

asyncpg/pool.py

@@ -9,6 +9,7 @@
 import functools
 import inspect
 import logging
+import sys
 import time
 import typing
 import warnings
@@ -24,7 +25,7 @@
 _Record = typing.TypeVar('_Record', bound=protocol.Record)
 
 
-class PoolConnectionProxyMeta(type):
+class _PoolConnectionProxyMeta(type):
 
     def __new__(mcls, name, bases, dct, *, wrap=False):
         if wrap:
@@ -65,7 +66,16 @@ def call_con_method(self, *args, **kwargs):
         return call_con_method
 
 
-class PoolConnectionProxy(connection._ConnectionProxy,
+if sys.version_info >= (3, 7):
+    PoolConnectionProxyMeta = _PoolConnectionProxyMeta
+else:
+    # see: https://github.com/python/typing/issues/449
+    class PoolConnectionProxyMeta(_PoolConnectionProxyMeta,
+                                  typing.GenericMeta):
+        ...
+
+
+class PoolConnectionProxy(connection._ConnectionProxy[_Record],
                           metaclass=PoolConnectionProxyMeta,
                           wrap=True):