Add support for shiny mode magic comment

Author: wch

rsconnect/shiny_express.py

@@ -4,8 +4,10 @@
 from __future__ import annotations
 
 import ast
-from pathlib import Path
 import re
+import sys
+from pathlib import Path
+from typing import Literal, cast
 
 __all__ = ("is_express_app",)
 
@@ -39,8 +41,16 @@ def is_express_app(app: str, app_dir: str | None) -> bool:
 
     try:
         # Read the file, parse it, and look for any imports of shiny.express.
-        with open(app_path) as f:
+        with open(app_path, encoding="utf-8") as f:
             content = f.read()
+
+        # Check for magic comment in the first 1000 characters
+        forced_mode = find_magic_comment_mode(content[:1000])
+        if forced_mode == "express":
+            return True
+        elif forced_mode == "core":
+            return False
+
         tree = ast.parse(content, app_path)
         detector = DetectShinyExpressVisitor()
         detector.visit(tree)
@@ -56,25 +66,50 @@ def __init__(self):
         super().__init__()
         self.found_shiny_express_import = False
 
-    def visit_Import(self, node: ast.Import):
+    def visit_Import(self, node: ast.Import) -> None:
         if any(alias.name == "shiny.express" for alias in node.names):
             self.found_shiny_express_import = True
 
-    def visit_ImportFrom(self, node: ast.ImportFrom):
+    def visit_ImportFrom(self, node: ast.ImportFrom) -> None:
         if node.module == "shiny.express":
             self.found_shiny_express_import = True
         elif node.module == "shiny" and any(alias.name == "express" for alias in node.names):
             self.found_shiny_express_import = True
 
     # Visit top-level nodes.
-    def visit_Module(self, node: ast.Module):
+    def visit_Module(self, node: ast.Module) -> None:
         super().generic_visit(node)
 
     # Don't recurse into any nodes, so the we'll only ever look at top-level nodes.
-    def generic_visit(self, node: ast.AST):
+    def generic_visit(self, node: ast.AST) -> None:
         pass
 
 
+def find_magic_comment_mode(content: str) -> Literal["core", "express"] | None:
+    """
+    Look for a magic comment of the form "# shiny_mode: express" or "# shiny_mode:
+    core".
+
+    If a line of the form "# shiny_mode: x" is found, where "x" is not "express" or
+    "core", then a message will be printed to stderr.
+
+    Returns
+    -------
+    :
+        `True` if Shiny Express comment is found, `False` if Shiny Core comment is
+        found, and `None` if no magic comment is found.
+    """
+    m = re.search(r"^#[ \t]*shiny_mode:[ \t]*(\S*)[ \t]*$", content, re.MULTILINE)
+    if m is not None:
+        shiny_mode = cast(str, m.group(1))
+        if shiny_mode in ("express", "core"):
+            return shiny_mode
+        else:
+            print(f'Invalid shiny_mode: "{shiny_mode}"', file=sys.stderr)
+
+    return None
+
+
 def escape_to_var_name(x: str) -> str:
     """
     Given a string, escape it to a valid Python variable name which contains