Implements Python error hint when importing from src.

Author: dom96

src/pyodide/python-entrypoint-helper.ts

@@ -95,6 +95,27 @@ export function setDoAnImport(
   };
 }
 
+function handleSrcImport(pyodide: Pyodide, e: any): never {
+  // Users may be expecting to import local modules via the `src` directory, which for a default
+  // project structure will fail. This code will add some extra info to the error message to help
+  // them fix it.
+  if (e.name === 'PythonError' && e.type === 'ModuleNotFoundError') {
+    pyodide.runPython(`
+      try:
+        import sys
+        exc = sys.last_value
+        if exc.name == "src":
+          exc.add_note(
+            "If your main module is inside the 'src' directory then your import " +
+            "statement shouldn't include a 'src.' prefix")
+        raise exc
+      finally:
+        del exc
+    `);
+  }
+  throw e;
+}
+
 async function pyimportMainModule(pyodide: Pyodide): Promise<PyModule> {
   if (!MAIN_MODULE_NAME.endsWith('.py')) {
     throw new PythonUserError(
@@ -243,6 +264,8 @@ function getMainModule(): Promise<PyModule> {
         return await enterJaegerSpan('pyimport_main_module', () =>
           pyimportMainModule(pyodide)
         );
+      } catch (e: any) {
+        handleSrcImport(pyodide, e);
       } finally {
         Limiter.finishStartup(LOADED_SNAPSHOT_TYPE);
       }
@@ -264,18 +287,26 @@ async function preparePython(): Promise<PyModule> {
   }
 }
 
-function doPyCallHelper(
+async function doPyCallHelper(
   relaxed: boolean,
   pyfunc: PyCallable,
   args: any[]
-): any {
-  if (pyfunc.callWithOptions) {
-    return pyfunc.callWithOptions({ relaxed, promising: true }, ...args);
-  }
-  if (relaxed) {
-    return pyfunc.callRelaxed(...args);
+): Promise<any> {
+  try {
+    if (pyfunc.callWithOptions) {
+      return await pyfunc.callWithOptions(
+        { relaxed, promising: true },
+        ...args
+      );
+    }
+    if (relaxed) {
+      return await pyfunc.callRelaxed(...args);
+    }
+    return await pyfunc(...args);
+  } catch (e: any) {
+    const pyodide = await getPyodide();
+    handleSrcImport(pyodide, e);
   }
-  return pyfunc(...args);
 }
 
 function doPyCall(pyfunc: PyCallable, args: any[]): any {