Optional boto3 (#115)

* Only require boto3 for WebSocket support

Author: jordaneremieff

docs/http.md

@@ -39,7 +39,7 @@ Additionally, any `Content-Type` header prefixed with `text/` is automatically e
 
 If the `Content-Encoding` header is set to `gzip`, then a binary response will be returned regardless of MIME type.
 
-## API
+## State machine
 
 The `HTTPCycle` is used by the adapter to communicate message events between the application and AWS. It is a state machine that handles the entire ASGI request and response cycle.
 

docs/lifespan.md

@@ -46,7 +46,7 @@ Lifespan support is automatically determined unless explicitly turned on or off.
 
 Defaults to `auto`.
 
-## API
+## State machine
 
 The `LifespanCycle` is a state machine that handles ASGI `lifespan` events intended to run before and after HTTP and WebSocket requests are handled. 
 

docs/websockets.md

@@ -235,7 +235,7 @@ sqlite://[file_path].db
 
     The file name or path to an sqlite3 database file. If one does not exist, then it will be created automatically.
 
-## API
+## State machine
 
 The `WebSocketCycle` is used by the adapter to communicate message events between the application and WebSocket client connections in API Gateway using a storage backend to persist the connection `scope`. It is a state machine that handles the ASGI request and response cycle for each individual message sent by a client.
 
@@ -245,7 +245,7 @@ The `WebSocketCycle` is used by the adapter to communicate message events betwee
     :docstring:
     :members: run receive send 
 
-#### Handling API Gateway events
+#### API Gateway events
 
 There are three WebSocket events sent by API Gateway for a WebSocket API connection. Each event requires returning a response immediately, and the information required to create the connection scope is only available in the initial `CONNECT` event. Messages are only sent in `MESSAGE` events that occur after the initial connection is established, and they do not include the details of the initial connect event. Due to the stateless nature of AWS Lambda, a storage backend is required to persist the WebSocket connection details for the duration of a client connection.
 

mangum/websocket.py

@@ -6,9 +6,11 @@
 from mangum.types import Scope
 from mangum.exceptions import WebSocketError, ConfigurationError
 
-
-import boto3
-from botocore.exceptions import ClientError
+try:
+    import boto3
+    from botocore.exceptions import ClientError
+except ImportError:  # pragma: no cover
+    boto3 = None
 
 
 @dataclass
@@ -20,6 +22,8 @@ class WebSocket:
     api_gateway_endpoint_url: str
 
     def __post_init__(self) -> None:
+        if boto3 is None:  # pragma: no cover
+            raise WebSocketError("boto3 must be installed to use WebSockets.")
         self.logger: logging.Logger = logging.getLogger("mangum.websocket")
         parsed_dsn = urlparse(self.dsn)
         if not any((parsed_dsn.hostname, parsed_dsn.path)):