add explicit load_backend function

urllib3/_async/connection.py

@@ -30,7 +30,7 @@
 )
 from urllib3.packages import six
 from ..util import ssl_ as ssl_util
-from .._backends import SyncBackend
+from .._backends import load_backend
 from .._backends._common import LoopAbort
 
 try:
@@ -306,13 +306,14 @@ class HTTP1Connection(object):
     #: ``[(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)]``
     default_socket_options = [(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)]
 
-    def __init__(self, host, port, backend=None,
+    def __init__(self, host, port,
+                 backend_name="sync", backend_args=None,
                  socket_options=_DEFAULT_SOCKET_OPTIONS,
                  source_address=None, tunnel_host=None, tunnel_port=None,
                  tunnel_headers=None):
         self.is_verified = False
-
-        self._backend = backend or SyncBackend()
+        self._backend_args = backend_args or {}
+        self._backend = load_backend(backend_name)(**self._backend_args)
         self._host = host
         self._port = port
         self._socket_options = (

urllib3/_backends/__init__.py

@@ -1,9 +1,54 @@
+import logging
+
 from urllib3.packages import six
-from .sync_backend import SyncBackend
 
-__all__ = ['SyncBackend']
 
-if six.PY3:
+def load_trio_backend():
+    if not six.PY3:
+        raise ValueError("trio backend requires Python 3")
     from .trio_backend import TrioBackend
+    return TrioBackend
+
+
+def load_twisted_backend():
+    if not six.PY3:
+        raise ValueError("twisted backend requires Python 3")
     from .twisted_backend import TwistedBackend
-    __all__ += ['TrioBackend', 'TwistedBackend']
+    return TwistedBackend
+
+
+def load_sync_backend():
+    from .sync_backend import SyncBackend
+    return SyncBackend
+
+
+BACKENDS = {
+    "sync": load_sync_backend,
+    "trio": load_trio_backend,
+    "twisted": load_twisted_backend,
+}
+
+
+def available_backends():
+    """
+    Returns a list of backend names that we are able to successfully import.
+    """
+    output = []
+    for backend_name, load_backend_fn in sorted(BACKENDS.items()):
+        try:
+            load_backend_fn()
+            output.append(backend_name)
+        except:  # noqa
+            logging.exception("{} not available".format(backend_name))
+    return output
+
+
+def load_backend(backend_name):
+    if backend_name in BACKENDS:
+        return BACKENDS[backend_name]()
+    else:
+        valid_backends = ", ".join(sorted(BACKENDS.keys()))
+        raise ValueError("Backend must be one of: {}".format(valid_backends))
+
+
+__all__ = ['available_backends', 'load_backend']