Add an AST browser extension to IDLE

The ASTBrowser extension adds an AST Browser command to the Tools menu.
It opens a window showing the abstract syntax tree of the editor content
(or, in the Shell, the current input), or of the selection if there is
one.  Selecting a node highlights the matching region in the editor and,
while the browser has focus, moves the editor cursor there; selecting
text or moving the cursor in the editor selects the innermost enclosing
node.  Double-clicking a node (or pressing Escape) hides the browser,
revealing the editor at the node.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Doc/library/idle.rst

@@ -310,6 +310,17 @@ Token Browser
    Double-click a row, or press :kbd:`Escape`,
    to hide the browser and return to the editor at the token.
 
+AST Browser
+   Open a window showing the abstract syntax tree of the editor content
+   (or, in the Shell, the current input),
+   or of the selection if there is one.
+   Selecting a node highlights the matching region in the editor
+   and moves the cursor there;
+   selecting text or moving the cursor in the editor
+   selects the innermost enclosing node.
+   Double-click a node, or press :kbd:`Escape`,
+   to hide the browser and return to the editor at the node.
+
 Options menu (Shell and Editor)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

Lib/idlelib/News3.txt

@@ -4,6 +4,12 @@ Released on 2026-10-01
 =========================
 
 
+gh-152942: Add an AST Browser to IDLE, opened from the Tools menu.  It
+shows the abstract syntax tree of the editor content, the Shell input,
+or the selection.  Selecting a node highlights the matching region in
+the editor, and selecting text in the editor selects the innermost
+enclosing node.  Patch by Serhiy Storchaka and Claude Code.
+
 gh-152941: Add a Token Browser to IDLE, opened from the new Tools menu.
 It lists the Python tokens of the editor content, the Shell input, or
 the selection, with token type names colored as by `python -m tokenize`.

Lib/idlelib/astbrowser.py

@@ -0,0 +1,331 @@
+"""An AST browser for IDLE.
+
+The ASTBrowser extension adds an "AST Browser" command to the Tools menu.
+It opens a window showing the abstract syntax tree of the editor content
+(or, in the Shell, the current input), or of the selection if there is
+one.  Selecting a node highlights the matching region in the editor and
+moves the editor cursor there; selecting text or moving the cursor in the
+editor selects the innermost matching node.  Double-clicking a node hides
+the browser (as does Escape), revealing the editor at the node.
+"""
+import ast
+
+from tkinter import Toplevel, TclError
+from tkinter import TOP, BOTTOM, LEFT, RIGHT, X, Y, BOTH, W, END, VERTICAL
+from tkinter import ttk
+
+from idlelib.config import idleConf
+
+# The editor tag that highlights the source of the selected nodes.
+TAG = "ASTBROWSER"
+
+
+class ASTBrowser:
+    """IDLE extension: open an AST browser from the Tools menu."""
+
+    menudefs = [
+        ('tools', [
+            ('_AST Browser', '<<ast-browser>>'),
+        ]),
+    ]
+
+    def __init__(self, editwin):
+        self.editwin = editwin
+        self.window = None
+
+    def ast_browser_event(self, event=None):
+        "Open the AST browser, or refresh the one already open."
+        if self.window is not None and self.window.winfo_exists():
+            self.window.refresh()
+        else:
+            self.window = ASTBrowserWindow(self.editwin.top, self.editwin.text)
+        return "break"
+
+
+class ASTBrowserWindow(Toplevel):
+    "Show the abstract syntax tree of a Text widget's content or selection."
+
+    def __init__(self, parent, text, *, _htest=False, _utest=False):
+        """Create the AST browser.
+
+        parent - the master widget of this window.
+        text - the editor Text widget to browse and drive.
+        _htest - bool; change box location when running htest.
+        _utest - bool; don't wait for user interaction when unit testing.
+        """
+        super().__init__(parent)
+        self.text = text
+        self.base = (1, 0)      # Editor index of the parsed region's start.
+        self.source_lines = []  # Lines of the parsed source (for byte->char).
+        self.ranges = {}        # Tree item id -> (start index, end index).
+        self.focused = False    # Whether the browser currently has the focus.
+        self.title("AST Browser")
+        self.protocol("WM_DELETE_WINDOW", self.hide)
+        self.bind("<Escape>", self.hide)
+        x = parent.winfo_rootx() + 20
+        y = parent.winfo_rooty() + (100 if _htest else 20)
+        self.geometry(f"640x480+{x}+{y}")
+        self.minsize(400, 300)
+
+        self.create_widgets()
+        self.configure_tag()
+        self.populate()
+        # Follow the editor and select the matching node.  <<Selection>> covers
+        # selection changes by keyboard or mouse (a generic <KeyRelease> is
+        # shadowed by IDLE's specific key bindings); the release events cover
+        # plain cursor moves that leave no selection.  These bindings live as
+        # long as the editor Text and are torn down together with it (and with
+        # this child window), so there is nothing to unbind.
+        text.bind("<<Selection>>", self.sync_from_editor, add="+")
+        text.bind("<KeyRelease>", self.sync_from_editor, add="+")
+        text.bind("<ButtonRelease-1>", self.sync_from_editor, add="+")
+        if not _utest:
+            self.deiconify()
+
+    def create_widgets(self):
+        bar = ttk.Frame(self, padding=(6, 6, 6, 0))
+        bar.pack(side=TOP, fill=X)
+        ttk.Button(bar, text="Refresh", command=self.populate).pack(side=LEFT)
+
+        self.status = ttk.Label(self, anchor=W, relief="sunken", padding=3)
+        self.status.pack(side=BOTTOM, fill=X)
+
+        frame = ttk.Frame(self, padding=6)
+        frame.pack(side=TOP, fill=BOTH, expand=True)
+        self.tree = ttk.Treeview(frame, show="tree", selectmode="extended")
+        vbar = ttk.Scrollbar(frame, orient=VERTICAL, command=self.tree.yview)
+        self.tree.configure(yscrollcommand=vbar.set)
+        vbar.pack(side=RIGHT, fill=Y)
+        self.tree.pack(side=LEFT, fill=BOTH, expand=True)
+        self.tree.bind("<<TreeviewSelect>>", self.select_nodes)
+        self.tree.bind("<Double-Button-1>", self.goto_node)
+        # The highlight is shown only while the browser has the focus.
+        self.bind("<FocusIn>", self.on_focus_in)
+        self.bind("<FocusOut>", self.on_focus_out)
+
+    def configure_tag(self):
+        "Give the highlight tag the theme's 'hit' colors."
+        try:
+            colors = idleConf.GetHighlight(idleConf.CurrentTheme(), 'hit')
+        except Exception:
+            colors = {'foreground': '#000000', 'background': '#ffff80'}
+        self.text.tag_configure(TAG, **colors)
+
+    def editor_selection(self):
+        "Return the editor's (first, last) selection, or ('', '') if none."
+        try:
+            # A plain Text raises without a selection; the IDLE editor
+            # returns an empty string instead.
+            return self.text.index("sel.first"), self.text.index("sel.last")
+        except TclError:
+            return "", ""
+
+    def editor_index(self, lineno, col):
+        "Map an AST (lineno, byte col) to an editor index, honoring the base."
+        if lineno <= len(self.source_lines):
+            # col_offset is a UTF-8 byte offset; convert it to a character one.
+            col = len(self.source_lines[lineno - 1].encode()[:col]
+                      .decode(errors="replace"))
+        base_row, base_col = self.base
+        if lineno == 1:
+            col += base_col
+        return f"{base_row + lineno - 1}.{col}"
+
+    def node_range(self, node):
+        "Return the (start, end) editor indices of a node, or None."
+        if getattr(node, "lineno", None) is None or node.end_lineno is None:
+            return None
+        return (self.editor_index(node.lineno, node.col_offset),
+                self.editor_index(node.end_lineno, node.end_col_offset))
+
+    def populate(self, event=None):
+        "Parse the content (or selection) and fill the tree."
+        self.hide_highlight()
+        self.tree.delete(*self.tree.get_children())
+        self.ranges.clear()
+        text = self.text
+        first, last = self.editor_selection()
+        if first and last:
+            scope = "selection"
+        else:
+            last = text.index("end-1c")
+            # In the Shell, browse just the current input, which starts at the
+            # "iomark"; a plain editor has no such mark.  IDLE's editor returns
+            # '' for a missing mark, while a plain Text raises TclError.
+            try:
+                first = text.index("iomark")
+            except TclError:
+                first = ""
+            if first:
+                scope = "input"
+            else:
+                first, scope = "1.0", "text"
+        self.base = tuple(int(i) for i in first.split("."))
+        source = text.get(first, last)
+        self.source_lines = source.splitlines()
+        error = count = None
+        try:
+            tree = ast.parse(source)
+        except SyntaxError as exc:
+            error = exc.msg
+        else:
+            count = self.add_node("", "", tree)
+        status = f"{count or 0} nodes in {scope}"
+        if error:
+            status += f"  —  incomplete: {error}"
+        self.status.configure(text=status)
+        self.sync_from_editor()
+
+    def add_node(self, parent_item, field, node):
+        "Insert a node and its descendants; return the number of nodes added."
+        inline = []           # Fields shown in this row: 'name=value'.
+        children = []         # Fields shown as child rows: (label, node).
+        for name, value in ast.iter_fields(node):
+            if isinstance(value, ast.AST):
+                if value._fields:
+                    children.append((name, value))
+                else:         # An operator or context, e.g. Add, Load.
+                    inline.append(f"{name}={type(value).__name__}")
+            elif isinstance(value, list):
+                nodes = [(f"{name}[{i}]", elt) for i, elt in enumerate(value)
+                         if isinstance(elt, ast.AST)]
+                if nodes:
+                    children += nodes
+                elif value:   # A non-empty list of scalars; drop empty ones.
+                    inline.append(f"{name}={value!r}")
+            elif value is not None or name == "value":  # Keep the None literal.
+                inline.append(f"{name}={value!r}")
+
+        label = type(node).__name__
+        if inline:
+            label += "(" + ", ".join(inline) + ")"
+        if field:
+            label = f"{field}: {label}"
+        item = self.tree.insert(parent_item, END, text=label, open=True)
+        if rng := self.node_range(node):
+            self.ranges[item] = rng
+
+        count = 1
+        for name, child in children:
+            count += self.add_node(item, name, child)
+        return count
+
+    def refresh(self):
+        "Re-parse the current range and bring the browser to the front."
+        self.populate()
+        self.deiconify()
+        self.lift()
+        self.focus_set()
+
+    def sync_from_editor(self, event=None):
+        "Select the innermost node matching the editor's selection or cursor."
+        first, last = self.editor_selection()
+        if not (first and last):
+            first = last = self.text.index("insert")
+        self.select_rows(self.enclosing_node(first, last))
+
+    def enclosing_node(self, first, last):
+        "Return [item] of the smallest node covering [first, last], or []."
+        best = None
+        for item, (start, end) in self.ranges.items():
+            if (self.text.compare(start, "<=", first)
+                    and self.text.compare(last, "<=", end)):
+                # Covering nodes are nested; keep the tightest (deepest) one.
+                if best is None or (self.text.compare(start, ">=", best[1])
+                                    and self.text.compare(end, "<=", best[2])):
+                    best = (item, start, end)
+        return [best[0]] if best else []
+
+    def select_rows(self, items):
+        "Select the given tree rows and reveal the first."
+        if items:
+            self.tree.selection_set(items)
+            self.tree.focus(items[0])
+            self.tree.see(items[0])
+
+    def select_nodes(self, event=None):
+        "Highlight the selected nodes and, while focused, follow with the cursor."
+        self.show_highlight(see=True)
+        # Move the editor cursor only when the browser drives the selection
+        # (it has the focus).  When the editor drives it, the browser is not
+        # focused, so the cursor is left alone and there is no feedback loop.
+        if self.focused:
+            self.move_cursor()
+
+    def show_highlight(self, see=False):
+        "Highlight the selected nodes' source while the browser has focus."
+        if not self.focused:  # Keep the editor clean while it is in use.
+            return
+        text = self.text
+        self.hide_highlight()
+        first = None
+        for item in self.tree.selection():
+            rng = self.ranges.get(item)
+            if rng and rng[0] != rng[1]:  # Skip nodes with no source span.
+                text.tag_add(TAG, *rng)
+                if first is None:
+                    first = rng[0]
+        text.tag_raise(TAG)
+        if see and first is not None:
+            text.see(first)
+
+    def on_focus_in(self, event=None):
+        "Restore the highlight when the browser regains focus."
+        self.focused = True
+        self.show_highlight()
+
+    def on_focus_out(self, event=None):
+        "Hide the highlight while the editor (or another window) has focus."
+        self.focused = False
+        self.hide_highlight()
+
+    def goto_node(self, event=None):
+        "Move the cursor to the double-clicked node and hide the browser."
+        self.move_cursor(self.tree.identify_row(event.y))
+        self.hide()
+        return "break"      # Suppress the default double-click handling.
+
+    def move_cursor(self, item=None):
+        "Move the editor cursor to a node (the first selected row by default)."
+        if item is None:
+            selection = self.tree.selection()
+            item = selection[0] if selection else None
+        rng = self.ranges.get(item)
+        if rng:
+            self.text.mark_set("insert", rng[0])
+            self.text.see(rng[0])
+
+    def hide(self, event=None):
+        """Withdraw the browser, revealing the editor and giving it focus.
+
+        Hiding our own window sidesteps the window manager's focus-stealing
+        prevention, which blocks a background editor window from being raised.
+        """
+        self.hide_highlight()
+        self.withdraw()
+        self.text.focus_set()
+
+    def hide_highlight(self, event=None):
+        try:
+            self.text.tag_remove(TAG, "1.0", "end")
+        except TclError:  # The editor may already be gone.
+            pass
+
+
+def _ast_browser(parent):  # htest #
+    "Set up a sample editor Text and open an AST browser on it."
+    from tkinter import Text
+    top = Toplevel(parent)
+    top.title("Sample editor")
+    text = Text(top, width=40, height=8)
+    text.insert("1.0", "import sys\n\ndef f(x):\n    return x + 1  # add one\n")
+    text.pack(fill=BOTH, expand=True)
+    return ASTBrowserWindow(top, text, _htest=True)
+
+
+if __name__ == "__main__":
+    from unittest import main
+    main('idlelib.idle_test.test_astbrowser', verbosity=2, exit=False)
+
+    from idlelib.idle_test.htest import run
+    run(_ast_browser)

Lib/idlelib/config-extensions.def

@@ -55,6 +55,12 @@ enable= 1
 [TokenBrowser_cfgBindings]
 token-browser=
 
+# A browser for the abstract syntax tree of the editor, from the Tools menu.
+[ASTBrowser]
+enable= 1
+[ASTBrowser_cfgBindings]
+ast-browser=
+
 # A fake extension for testing and example purposes.  When enabled and
 # invoked, inserts or deletes z-text at beginning of every line.
 [ZzDummy]

Lib/idlelib/editor.py

@@ -1155,6 +1155,7 @@ def get_standard_extension_names(self):
 
     extfiles = {  # Map built-in config-extension section names to file names.
         'TokenBrowser': 'tokenbrowser',
+        'ASTBrowser': 'astbrowser',
         'ZzDummy': 'zzdummy',
         }