Python: Replace import magic with an auto-loader

As discussed on Reddit, the magic import logic is not very tool friendly and a little too magic perhaps. Instead, this patch introduces an automatic loader (`slint.loader`), which can traverse `sys.path` and lazily load `.slint` files by attribute lookup.

Closes #4856

api/python/README.md

@@ -74,9 +74,8 @@ export component AppWindow inherits Window {
 
 ```python
 import slint
-import appwindow_slint
 
-class App(appwindow_slint.AppWindow):
+class App(slint.loader.appwindow.AppWindow):
     @slint.callback
     def request_increase_value(self):
         self.counter = self.counter + 1
@@ -93,7 +92,7 @@ app.run()
 
 The following example shows how to instantiate a Slint component in Python:
 
-**`ui.slint`**
+**`app.slint`**
 
 ```slint
 export component MainWindow inherits Window {
@@ -111,21 +110,25 @@ export component MainWindow inherits Window {
 The exported component is exposed as a Python class. To access this class, you have two
 options:
 
-1. Call `slint.load_file("ui.slint")`. The returned object is a [namespace](https://docs.python.org/3/library/types.html#types.SimpleNamespace),
+1. Call `slint.load_file("app.slint")`. The returned object is a [namespace](https://docs.python.org/3/library/types.html#types.SimpleNamespace),
    that provides the `MainWindow` class:
    ```python
    import slint
-   components = slint.load_file("ui.slint")
+   components = slint.load_file("app.slint")
    main_window = components.MainWindow()
    ```
 
-2. Import the `.slint` file as module by treating it like a Python module where the `.slint` extension is replaced with `_slint`:
+2. Use Slint's auto-loader, which lazily loads `.slint` files from `sys.path`:
    ```python
-   import slint # needs to come first
-   from ui_slint import MainWindow
-   main_window = MainWindow()
+   import slint
+   # Look for for `app.slint` in `sys.path`:
+   main_window = slint.loader.app.MainWindow()
    ```
 
+   Any attribute lookup in `slint.loader` is searched for in `sys.path`. If a directory with the name exists, it is returned as a loader object, and subsequent
+   attribute lookups follow the same logic. If the name matches a file with the `.slint` extension, it is automatically loaded with `load_file` and the
+   [namespace](https://docs.python.org/3/library/types.html#types.SimpleNamespace) is returned.
+
 ### Accessing Properties
 
 [Properties](../slint/src/language/syntax/properties) declared as `out` or `in-out` in `.slint` files are visible as  properties on the component instance.

api/python/pyproject.toml

@@ -7,7 +7,7 @@ build-backend = "maturin"
 
 [project]
 name = "slint"
-version = "1.6.0a5"
+version = "1.6.0a6"
 requires-python = ">= 3.10"
 authors = [
     {name = "Slint Team", email = "info@slint.dev"},

api/python/slint/__init__.py

@@ -189,30 +189,31 @@ def load_file(path, quiet=False, style=None, include_paths=None, library_paths=N
     return module
 
 
-class SlintModuleLoader:
-    def create_module(self, spec):
+class SlintAutoLoader:
+    def __init__(self, base_dir=None):
+        if base_dir:
+            self.local_dirs = [base_dir]
+        else:
+            self.local_dirs = None
+
+    def __getattr__(self, name):
+        for path in self.local_dirs or sys.path:
+            dir_candidate = os.path.join(path, name)
+            if os.path.isdir(dir_candidate):
+                loader = SlintAutoLoader(dir_candidate)
+                setattr(self, name, loader)
+                return loader
+
+            file_candidate = dir_candidate + ".slint"
+            if os.path.isfile(file_candidate):
+                type_namespace = load_file(file_candidate)
+                setattr(self, name, type_namespace)
+                return type_namespace
+
         return None
 
-    def exec_module(self, module):
-        m = load_file(module.__name__)
-        module.__dict__.update(m.__dict__)
 
-
-class SlintModuleFinder:
-    def find_spec(self, name, path, target=None):
-        if "." in name:
-            return None
-
-        if not name.endswith("_slint"):
-            return None
-
-        candidate_filename = name.removesuffix("_slint") + ".slint"
-
-        for path in sys.path:
-            candidate = os.path.join(path, candidate_filename)
-            if os.path.exists(candidate):
-                return ModuleSpec(os.path.realpath(candidate), SlintModuleLoader())
-        return None
+loader = SlintAutoLoader()
 
 
 def _callback_decorator(callable, info):
@@ -235,8 +236,6 @@ def callback(global_name=None, name=None):
         return lambda callback: _callback_decorator(callback, info)
 
 
-sys.meta_path.append(SlintModuleFinder())
-
 Image = native.PyImage
 Color = native.PyColor
 Brush = native.PyBrush

api/python/tests/test_loader.py

@@ -3,25 +3,23 @@
 
 import pytest
 from slint import slint as native
+from slint import loader
 import sys
 import os
 
 
 def test_magic_import():
-    import test_load_file_slint as compiledmodule
-    instance = compiledmodule.App()
+    instance = loader.test_load_file.App()
     del instance
 
 
 def test_magic_import_path():
     oldsyspath = sys.path
-    with pytest.raises(ModuleNotFoundError, match="No module named 'printerdemo_slint'"):
-        import printerdemo_slint
+    assert loader.printerdemo == None
     try:
         sys.path.append(os.path.join(os.path.dirname(__file__),
-                        "..", "..", "..", "examples", "printerdemo", "ui"))
-        import printerdemo_slint
-        instance = printerdemo_slint.MainWindow()
+                        "..", "..", ".."))
+        instance = loader.examples.printerdemo.ui.printerdemo.MainWindow()
         del instance
     finally:
         sys.path = oldsyspath