Assert that we have frame information.

@willmcgugan I went with the asserts because the tone of the code tells me that we are kind of certain that the current and previous frames always exist. Should the function be refactored to handle None for the previous frame (or the current & previous frames) by writing to the buffer without caller information, for example?

mypy.txt

@@ -3,9 +3,7 @@ src/textual/_two_way_dict.py:41: error: Incompatible return value type (got "Opt
 src/textual/_two_way_dict.py:52: error: Incompatible return value type (got "Optional[Key]", expected "Key")  [return-value]
 src/textual/devtools/service.py:10: error: Skipping analyzing "msgpack": module is installed, but missing library stubs or py.typed marker  [import]
 src/textual/devtools/client.py:13: error: Skipping analyzing "msgpack": module is installed, but missing library stubs or py.typed marker  [import]
-src/textual/devtools/redirect_output.py:42: error: Item "None" of "Optional[FrameType]" has no attribute "f_back"  [union-attr]
-src/textual/devtools/redirect_output.py:43: error: Argument 1 to "getframeinfo" has incompatible type "Union[FrameType, None, Any]"; expected "Union[FrameType, TracebackType]"  [arg-type]
-src/textual/devtools/redirect_output.py:45: error: Argument "caller" to "DevtoolsLog" has incompatible type "Traceback"; expected "FrameInfo"  [arg-type]
+src/textual/devtools/redirect_output.py:48: error: Argument "caller" to "DevtoolsLog" has incompatible type "Traceback"; expected "FrameInfo"  [arg-type]
 src/textual/__init__.py:27: error: Library stubs not installed for "pkg_resources"  [import]
 src/textual/__init__.py:27: note: Hint: "python3 -m pip install types-setuptools"
 src/textual/__init__.py:27: note: (or run "mypy --install-types" to install all missing stub packages)
@@ -92,4 +90,4 @@ src/textual/demo.py:221: error: "App[Any]" has no attribute "add_note"  [attr-de
 src/textual/demo.py:276: error: "App[Any]" has no attribute "add_note"  [attr-defined]
 src/textual/cli/previews/easing.py:102: error: Argument "easing" to "animate" of "App" has incompatible type "Optional[str]"; expected "Union[Callable[[float], float], str]"  [arg-type]
 src/textual/cli/previews/easing.py:111: error: "MessageTarget" has no attribute "id"  [attr-defined]
-Found 84 errors in 28 files (checked 152 source files)
+Found 82 errors in 28 files (checked 152 source files)

src/textual/devtools/redirect_output.py

@@ -39,7 +39,10 @@ def write(self, string: str) -> None:
         if not self.devtools.is_connected:
             return
 
-        previous_frame = inspect.currentframe().f_back
+        current_frame = inspect.currentframe()
+        assert current_frame is not None
+        previous_frame = current_frame.f_back
+        assert previous_frame is not None
         caller = inspect.getframeinfo(previous_frame)
 
         self._buffer.append(DevtoolsLog(string, caller=caller))

src/textual/devtools/service.py

@@ -5,10 +5,10 @@
 import json
 import pickle
 from json import JSONDecodeError
-from typing import Any, cast
+from typing import Any
 
 import msgpack
-from aiohttp import WSMessage, WSMsgType
+from aiohttp import WSMsgType
 from aiohttp.abc import Request
 from aiohttp.web_ws import WebSocketResponse
 from rich.console import Console