Update messaging/typing/comments

Author: holtskinner

README.md

@@ -40,18 +40,23 @@ If you prefer to use pip, the standard Python package installer, you can install
 pip install a2a-sdk
 ```
 
-### Optional
+### Optional Dependencies
 
 #### FastAPI-based A2A Server App
 
-Using `a2a.server.apps.A2AFastAPIApplication` requires the FastAPI package, which is an optional dependency.
-With `uv`, it can be specified as follows:
+If you plan to build a server application using the `A2AFastAPIApplication` helper, you will need to install the `fastapi` package. This is an optional dependency.
+
+You can install it with the `fastapi` extra:
 
 ```shell
+# Using uv
 uv add a2a-sdk[fastapi]
+
+# Using pip
+pip install "a2a-sdk[fastapi]"
 ```
 
-Alternatively, a preferred version of the `fastapi` package can be added directly to the project:
+Alternatively, if you manage your dependencies separately, you can add `fastapi` to your project directly:
 
 ```shell
 uv add fastapi

src/a2a/server/apps/jsonrpc/fastapi_import_helpers.py

@@ -1,8 +1,13 @@
 """Helper functions for handling optional FastAPI package imports."""
 
-try:
+import sys
+
+
+_FASTAPI_INSTALLED = 'fastapi' in sys.modules
+
+if _FASTAPI_INSTALLED:
     from fastapi import FastAPI, Request, Response
-except ImportError:
+else:
     _FASTAPI_DEPENDENCY_ERROR_MSG = """
     The A2A Python SDK FastAPI app requires the FastAPI package, which
     is an optional dependency. To fix this issue, please add the fastapi
@@ -15,18 +20,18 @@
         uv add a2a-sdk[fastapi]
     """
 
-    class _DummyFastAPIClasses:
-        """Parent class for dummy fastapi.* class declarations."""
+    class _OptionalDependencyPlaceholder:
+        """Base class for placeholder fastapi.* declarations."""
 
-        def __init__(self) -> None:
-            """Raises ImportError when initiating a dummy fastapi.* instance."""
+        def __init__(self, *args, **kwargs):
+            """Raises ImportError when an instance is created."""
             raise ImportError(_FASTAPI_DEPENDENCY_ERROR_MSG)
 
-    class FastAPI(_DummyFastAPIClasses):  # type: ignore[no-redef]
-        """A dummy fastapi.FastAPI declaration."""
+    class FastAPI(_OptionalDependencyPlaceholder):
+        """A placeholder for fastapi.FastAPI."""
 
-    class Request(_DummyFastAPIClasses):  # type: ignore[no-redef]
-        """A dummy fastapi.Request declaration."""
+    class Request(_OptionalDependencyPlaceholder):
+        """A placeholder for fastapi.Request."""
 
-    class Response(_DummyFastAPIClasses):  # type: ignore[no-redef]
-        """A dummy fastapi.Response declaration."""
+    class Response(_OptionalDependencyPlaceholder):
+        """A placeholder for fastapi.Response."""