Relative string CSS paths

sandbox/darren/just_a_box.py

@@ -44,5 +44,5 @@ class JustABox(App):
 
 
 if __name__ == "__main__":
-    app = JustABox(css_path="just_a_box.css", watch_css=True)
+    app = JustABox(css_path="../darren/just_a_box.css", watch_css=True)
     app.run()

src/textual/_path.py

@@ -0,0 +1,28 @@
+import inspect
+import sys
+from pathlib import Path
+
+
+def _make_path_object_relative(path: str, obj: object) -> Path:
+    """Given a path as a string convert it, if possible, to a Path that is relative to a given Python object.
+    If the path string is already absolute, it will simply be converted to a Path object.
+    Used, for example, to return the path of a CSS file relative to a Textual App instance.
+
+    Args:
+        path (str): A path, as a string.
+        obj (object): A Python object.
+
+    Returns:
+        Path: A resolved Path object, relative to obj
+    """
+    path = Path(path)
+
+    # If the path supplied by the user is absolute, we can use it directly
+    if path.is_absolute():
+        return path
+
+    # Otherwise (relative path), resolve it relative to obj...
+    subclass_module = sys.modules[obj.__module__]
+    subclass_path = Path(inspect.getfile(subclass_module))
+    resolved_path = (subclass_path.parent / path).resolve()
+    return resolved_path

src/textual/app.py

@@ -9,8 +9,9 @@ import sys
 import warnings
 from contextlib import redirect_stdout, redirect_stderr
 from datetime import datetime
-from pathlib import PurePath
+from pathlib import PurePath, Path
 from time import perf_counter
+from types import ModuleType
 from typing import (
     TYPE_CHECKING,
     Any,
@@ -25,6 +26,7 @@ from typing import (
 from weakref import WeakSet, WeakValueDictionary
 
 from ._ansi_sequences import SYNC_END, SYNC_START
+from ._path import _make_path_object_relative
 
 if sys.version_info >= (3, 8):
     from typing import Literal
@@ -147,7 +149,7 @@ class App(Generic[ReturnType], DOMNode):
     CSS = """
     App {
         background: $background;
-        color: $text-background;             
+        color: $text-background;
     }
     """
 
@@ -225,7 +227,12 @@ class App(Generic[ReturnType], DOMNode):
 
         self.stylesheet = Stylesheet(variables=self.get_css_variables())
         self._require_stylesheet_update: set[DOMNode] = set()
-        self.css_path = css_path or self.CSS_PATH
+
+        css_path = css_path or self.CSS_PATH
+        if isinstance(css_path, str):
+            css_path = _make_path_object_relative(css_path, self) if css_path else None
+
+        self.css_path = css_path
 
         self._registry: WeakSet[DOMNode] = WeakSet()
 

src/textual/file_monitor.py

@@ -1,4 +1,3 @@
-from __future__ import annotations
 import os
 from pathlib import PurePath
 from typing import Callable
@@ -12,7 +11,7 @@ from ._callback import invoke
 class FileMonitor:
     """Monitors a file for changes and invokes a callback when it does."""
 
-    def __init__(self, path: str | PurePath, callback: Callable) -> None:
+    def __init__(self, path: PurePath, callback: Callable) -> None:
         self.path = path
         self.callback = callback
         self._modified = self._get_modified()