fix: conversion of ipynb files with multiple definitions (#3458)

The `NameTransformer` wasn't visiting node descendants, leading to
incomplete transformations.

Before, when remapping `x` to a local:

```python
x = 1
y = foo(x + 1)
```

was yielding

```python
x = 1
y = foo(x + 1)
```

With this change, it now yields

```python
_x = 1
y = foo(_x + 1)
```

This change also modifies an old test that was testing that the
`NameTransformer` respected scoping logic. But it never actually did, or
rather only did so because of a bug in which it wasn't visiting entire
subtrees. Nothing relies on the `NameTransformer` respecting scoping in
our codebase.

marimo/_ast/transformers.py

@@ -7,11 +7,18 @@
 
 class NameTransformer(ast.NodeTransformer):
     def __init__(self, name_substitutions: dict[str, str]) -> None:
+        """Remaps names in an AST.
+
+        Naively remaps all occurrences of names in an AST, given a substitution
+        dict mapping old names to new names. In particular does not take
+        scoping into account.
+        """
         self._name_substitutions = name_substitutions
         self.made_changes = False
         super().__init__()
 
     def visit_Name(self, node: ast.Name) -> ast.Name:
+        self.generic_visit(node)
         if node.id in self._name_substitutions:
             self.made_changes = True
             return ast.Name(
@@ -20,6 +27,7 @@ def visit_Name(self, node: ast.Name) -> ast.Name:
         return node
 
     def visit_FunctionDef(self, node: ast.FunctionDef) -> ast.FunctionDef:
+        self.generic_visit(node)
         if node.name in self._name_substitutions:
             self.made_changes = True
             return ast.FunctionDef(
@@ -31,6 +39,7 @@ def visit_FunctionDef(self, node: ast.FunctionDef) -> ast.FunctionDef:
         return node
 
     def visit_ClassDef(self, node: ast.ClassDef) -> ast.ClassDef:
+        self.generic_visit(node)
         if node.name in self._name_substitutions:
             self.made_changes = True
             return ast.ClassDef(
@@ -42,6 +51,7 @@ def visit_ClassDef(self, node: ast.ClassDef) -> ast.ClassDef:
         return node
 
     def visit_Assign(self, node: ast.Assign) -> ast.Assign:
+        self.generic_visit(node)
         new_targets: list[Any] = []
         for target in node.targets:
             if (

tests/_ast/test_transformers.py

@@ -12,7 +12,10 @@
     sys.version_info < (3, 9), reason="Feature not supported in python 3.8"
 )
 def test_name_transformer() -> None:
-    # Test code
+    # Name transformer should naively remap all occurrences of names in an AST,
+    # without taking scoping into account.
+    #
+    # It does not transform attributes.
     code = """
 def old_function():
     old_variable = 42
@@ -47,8 +50,8 @@ def __init__(self):
     # Expected transformed code
     expected_code = """
 def new_function():
-    old_variable = 42
-    return old_variable
+    new_variable = 42
+    return new_variable
 
 class NewClass: