Logging handler (#2151)

* logging handler

* changelog

* remove logging experiment

* handler

* fix

* docs for logging handler

CHANGELOG.md

@@ -29,6 +29,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
   - `Tree`: `clear`, `reset`
 - Screens with alpha in their background color will now blend with the background. https://github.com/Textualize/textual/pull/2139
 - Added "thick" border style. https://github.com/Textualize/textual/pull/2139
+- message_pump.app will now set the active app if it is not already set.
 
 ### Added
 
@@ -36,7 +37,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Added scroll_end switch to TextLog.write https://github.com/Textualize/textual/pull/2127
 - Added Screen.ModalScreen which prevents App from handling bindings. https://github.com/Textualize/textual/pull/2139
 - Added TEXTUAL_LOG env var which should be a path that Textual will write verbose logs to (textual devtools is generally preferred) https://github.com/Textualize/textual/pull/2148
-
+- Added textual.logging.TextualHandler logging handler
 
 ## [0.16.0] - 2023-03-22
 

docs/api/logging.md

@@ -0,0 +1 @@
+::: textual.logging

docs/guide/devtools.md

@@ -120,5 +120,37 @@ class LogApp(App):
 
 if __name__ == "__main__":
     LogApp().run()
-
+```
+
+### Logging handler
+
+Textual has a [logging handler][textual.logging.TextualHandler] which will write anything logged via the builtin logging library to the devtools.
+This may be useful if you have a third-party library that uses the logging module, and you want to see those logs with Textual logs.
+
+!!! note
+
+    The logging library works with strings only, so you won't be able to log Rich renderables such as `self.tree` with the logging handler.
+
+Here's an example of configuring logging to use the `TextualHandler`.
+
+```python
+import logging
+from textual.app import App
+from textual.logging import TextualHandler
+
+logging.basicConfig(
+    level="NOTSET",
+    handlers=[TextualHandler()],
+)
+
+
+class LogApp(App):
+    """Using logging with Textual."""
+
+    def on_mount(self) -> None:
+        logging.debug("Logged via TextualHandler")
+
+
+if __name__ == "__main__":
+    LogApp().run()
 ```

mkdocs-nav.yml

@@ -169,6 +169,7 @@ nav:
     - "api/list_item.md"
     - "api/list_view.md"
     - "api/loading_indicator.md"
+    - "api/logging.md"
     - "api/markdown_viewer.md"
     - "api/markdown.md"
     - "api/message_pump.md"

src/textual/__init__.py

@@ -142,6 +142,11 @@ class Logger:
         """Logs system information."""
         return Logger(self._log, LogGroup.SYSTEM)
 
+    @property
+    def logging(self) -> Logger:
+        """Logs from stdlib logging module."""
+        return Logger(self._log, LogGroup.LOGGING)
+
 
 log = Logger(None)
 

src/textual/_log.py

@@ -12,6 +12,7 @@ class LogGroup(Enum):
     ERROR = 5
     PRINT = 6
     SYSTEM = 7
+    LOGGING = 8
 
 
 class LogVerbosity(Enum):

src/textual/logging.py

@@ -0,0 +1,32 @@
+import sys
+from logging import Handler, LogRecord
+
+from ._context import active_app
+
+
+class TextualHandler(Handler):
+    """A Logging handler for Textual apps."""
+
+    def __init__(self, stderr: bool = True, stdout: bool = False) -> None:
+        """Initialize a Textual logging handler.
+
+        Args:
+            stderr: Log to stderr when there is no active app.
+            stdout: Log to stdout when there is not active app.
+        """
+        super().__init__()
+        self._stderr = stderr
+        self._stdout = stdout
+
+    def emit(self, record: LogRecord) -> None:
+        """Invoked by logging."""
+        message = self.format(record)
+        try:
+            app = active_app.get()
+        except LookupError:
+            if self._stderr:
+                print(message, file=sys.stderr)
+            elif self._stdout:
+                print(message, file=sys.stdout)
+        else:
+            app.log.logging(message)

src/textual/message_pump.py

@@ -156,17 +156,26 @@ class MessagePump(metaclass=MessagePumpMeta):
         try:
             return active_app.get()
         except LookupError:
-            raise NoActiveAppError()
+            from .app import App
+
+            node: MessagePump | None = self
+            while not isinstance(node, App):
+                if node is None:
+                    raise NoActiveAppError()
+                node = node._parent
+            active_app.set(node)
+            return node
 
     @property
     def is_parent_active(self) -> bool:
+        """Is the parent active?"""
         return bool(
             self._parent and not self._parent._closed and not self._parent._closing
         )
 
     @property
     def is_running(self) -> bool:
-        """bool: Is the message pump running (potentially processing messages)."""
+        """Is the message pump running (potentially processing messages)."""
         return self._running
 
     @property