Enable relative imports in Express apps

CHANGELOG.md

@@ -30,6 +30,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * `@render.data_frame`'s `<ID>.cell_selection()` no longer returns a `None` value and now always returns a dictionary containing both the `rows` and `cols` keys. This is done to achieve more consistent author code when working with cell selection. When the value's `type="none"`, both `rows` and `cols` are empty tuples. When `type="row"`, `cols` represents all column numbers of the data. In the future, when `type="col"`, `rows` will represent all row numbers of the data. These extra values are not available in `input.<ID>_cell_selection()` as they are independent of cells being selected and are removed to reduce information being sent to and from the browser. (#1376)
 
+* Relative imports, like `from . import utils`, now can be used in Shiny Express apps. (#1464)
 
 ### Bug fixes
 

shiny/express/_run.py

@@ -1,9 +1,13 @@
 from __future__ import annotations
 
 import ast
+import importlib.abc
+import importlib.util
 import sys
+import types
+from importlib.machinery import ModuleSpec
 from pathlib import Path
-from typing import Mapping, cast
+from typing import Mapping, Sequence, cast
 
 from htmltools import Tag, TagList
 
@@ -27,11 +31,19 @@
     "wrap_express_app",
 )
 
+# Mapping from package name to file path of app. When running multiple concurrent apps
+# (as is possible with shinylive), we need to give each one a unique package name.
+package_filepath_map: dict[str, Path] = {}
+
 
 @no_example()
 def wrap_express_app(file: Path) -> App:
     """Wrap a Shiny Express mode app into a Shiny `App` object.
 
+    This also creates a Python package for the app named something like
+    `shiny_express_app_0`. This package is required for relative imports to work, as in
+    `from . import utils`.
+
     Parameters
     ----------
     file
@@ -42,6 +54,64 @@ def wrap_express_app(file: Path) -> App:
     :
         A :class:`shiny.App` object.
     """
+    package_name = f"shiny_express_app_{len(package_filepath_map)}"
+    package_filepath_map[package_name] = file
+
+    # Importing the module triggers the ShinyExpressAppImportFinder and
+    # ShinyExpressAppLoader.
+    app_module = importlib.import_module(package_name)
+    return app_module.app
+
+
+# ======================================================================================
+# Import hook to load Shiny Express app as a package
+# ======================================================================================
+class ShinyExpressAppImportFinder(importlib.abc.MetaPathFinder):
+    def __init__(self):
+        self.loaded_modules: dict[str, ModuleSpec] = {}
+
+    def find_spec(
+        self,
+        fullname: str,
+        path: Sequence[str] | None,
+        target: types.ModuleType | None = None,
+    ) -> ModuleSpec | None:
+        if fullname in self.loaded_modules:
+            return self.loaded_modules[fullname]
+
+        if fullname in package_filepath_map:
+            app_path = package_filepath_map[fullname]
+            app_dir = str(app_path.parent)
+            spec = importlib.util.spec_from_loader(
+                fullname, ShinyExpressAppLoader(), origin=app_dir
+            )
+            if spec is None:
+                return None
+
+            spec.submodule_search_locations = [app_dir]
+            self.loaded_modules[fullname] = spec
+            return spec
+
+
+class ShinyExpressAppLoader(importlib.abc.Loader):
+    def create_module(self, spec: ModuleSpec):
+        my_module = types.ModuleType(spec.name)
+        return my_module
+
+    def exec_module(self, module: types.ModuleType) -> None:
+        module.app = create_express_app(  # pyright: ignore[reportAttributeAccessIssue]
+            package_filepath_map[module.__name__], module.__name__
+        )
+
+
+sys.meta_path.insert(0, ShinyExpressAppImportFinder())
+# ======================================================================================
+
+
+# This is invoked from the ShinyExpressAppLoader.exec_module() method above. It creates
+# the App object from the Shiny Express app file.
+@no_example()
+def create_express_app(file: Path, package_name: str) -> App:
 
     file = file.resolve()
 
@@ -59,14 +129,14 @@ def wrap_express_app(file: Path) -> App:
             # catch them here and convert them to a different type of error, because uvicorn
             # specifically catches AttributeErrors and prints an error message that is
             # misleading for Shiny Express. https://github.com/posit-dev/py-shiny/issues/937
-            app_ui = run_express(file).tagify()
+            app_ui = run_express(file, package_name).tagify()
 
     except AttributeError as e:
         raise RuntimeError(e) from e
 
     def express_server(input: Inputs, output: Outputs, session: Session):
         try:
-            run_express(file)
+            run_express(file, package_name)
 
         except Exception:
             import traceback
@@ -93,7 +163,21 @@ def express_server(input: Inputs, output: Outputs, session: Session):
     return app
 
 
-def run_express(file: Path) -> Tag | TagList:
+def run_express(file: Path, package_name: str | None = None) -> Tag | TagList:
+    """
+    Run the code in a Shiny Express app file and return the UI. This is to be run in
+    both the UI-rendering phase and the server-rendering phase of a Shiny Express app.
+    When used in the server-rendering phase, the returned UI should simply be ignored.
+
+    Parameters
+    ----------
+    file
+        The path to the file containing the Shiny Express application.
+    package_name
+        The name of the package for the app. This is generated by `wrap_express_app()`
+        and should be something like "shiny_express_app_0". The purpose of this is to
+        allow relative imports in the app code.
+    """
     with open(file, encoding="utf-8") as f:
         content = f.read()
 
@@ -118,6 +202,8 @@ def set_result(x: object):
 
         var_context: dict[str, object] = {
             "__file__": file_path,
+            "__name__": "app",
+            "__package__": package_name,
             expressify_decorator_func_name: _expressify_decorator_function_def,
             "input": InputNotImportedShim(),
         }

shiny/express/app.py

@@ -2,13 +2,12 @@
 
 from pathlib import Path
 
-from .._app import App
 from ._run import wrap_express_app
 from ._utils import unescape_from_var_name
 
 
 # If someone requests shiny.express.app:_2f_path_2f_to_2f_app_2e_py, then we will call
 # wrap_express_app(Path("/path/to/app.py")) and return the result.
-def __getattr__(name: str) -> App:
+def __getattr__(name: str) -> object:
     name = unescape_from_var_name(name)
     return wrap_express_app(Path(name))