use getattr to lazy load aiohttp and trio (Azure#15878)

* use getattr to lazy load aiohttp and trio

Author: xiangyan99

sdk/core/azure-core/azure/core/pipeline/transport/__init__.py

@@ -24,6 +24,7 @@
 #
 # --------------------------------------------------------------------------
 
+import sys
 from ._base import HttpTransport, HttpRequest, HttpResponse
 from ._requests_basic import RequestsTransport, RequestsTransportResponse
 
@@ -35,8 +36,7 @@
     'RequestsTransportResponse',
 ]
 
-#pylint: disable=unused-import
-
+# pylint: disable=unused-import, redefined-outer-name
 try:
     from ._base_async import AsyncHttpTransport, AsyncHttpResponse
     from ._requests_asyncio import AsyncioRequestsTransport, AsyncioRequestsTransportResponse
@@ -47,22 +47,61 @@
         'AsyncioRequestsTransportResponse'
     ])
 
-    try:
-        from ._requests_trio import TrioRequestsTransport, TrioRequestsTransportResponse
+    if sys.version_info >= (3, 7):
         __all__.extend([
             'TrioRequestsTransport',
-            'TrioRequestsTransportResponse'
-        ])
-    except ImportError:
-        pass  # Trio not installed
-
-    try:
-        from ._aiohttp import AioHttpTransport, AioHttpTransportResponse
-        __all__.extend([
+            'TrioRequestsTransportResponse',
             'AioHttpTransport',
             'AioHttpTransportResponse',
         ])
-    except ImportError:
-        pass  # Aiohttp not installed
+
+        def __dir__():
+            return __all__
+
+        def __getattr__(name):
+            if name == 'AioHttpTransport':
+                try:
+                    from ._aiohttp import AioHttpTransport
+                    return AioHttpTransport
+                except ImportError:
+                    raise ImportError("aiohttp package is not installed")
+            if name == 'AioHttpTransportResponse':
+                try:
+                    from ._aiohttp import AioHttpTransportResponse
+                    return AioHttpTransportResponse
+                except ImportError:
+                    raise ImportError("aiohttp package is not installed")
+            if name == 'TrioRequestsTransport':
+                try:
+                    from ._requests_trio import TrioRequestsTransport
+                    return TrioRequestsTransport
+                except ImportError:
+                    raise ImportError("trio package is not installed")
+            if name == 'TrioRequestsTransportResponse':
+                try:
+                    from ._requests_trio import TrioRequestsTransportResponse
+                    return TrioRequestsTransportResponse
+                except ImportError:
+                    raise ImportError("trio package is not installed")
+            return name
+
+    else:
+        try:
+            from ._requests_trio import TrioRequestsTransport, TrioRequestsTransportResponse
+            __all__.extend([
+                'TrioRequestsTransport',
+                'TrioRequestsTransportResponse'
+            ])
+        except ImportError:
+            pass  # Trio not installed
+
+        try:
+            from ._aiohttp import AioHttpTransport, AioHttpTransportResponse
+            __all__.extend([
+                'AioHttpTransport',
+                'AioHttpTransportResponse',
+            ])
+        except ImportError:
+            pass  # Aiohttp not installed
 except (ImportError, SyntaxError):
     pass  # Asynchronous pipelines not supported.