bpo-37903: IDLE: Shell sidebar with prompts (GH-22682)

The first followup will change shell indents to spaces.
More are expected.

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

Lib/idlelib/idle_test/tkinter_testing_utils.py

@@ -0,0 +1,56 @@
+"""Utilities for testing with Tkinter"""
+import functools
+
+
+def run_in_tk_mainloop(test_method):
+    """Decorator for running a test method with a real Tk mainloop.
+
+    This starts a Tk mainloop before running the test, and stops it
+    at the end. This is faster and more robust than the common
+    alternative method of calling .update() and/or .update_idletasks().
+
+    Test methods using this must be written as generator functions,
+    using "yield" to allow the mainloop to process events and "after"
+    callbacks, and then continue the test from that point.
+
+    This also assumes that the test class has a .root attribute,
+    which is a tkinter.Tk object.
+
+    For example (from test_sidebar.py):
+
+    @run_test_with_tk_mainloop
+    def test_single_empty_input(self):
+        self.do_input('\n')
+        yield
+        self.assert_sidebar_lines_end_with(['>>>', '>>>'])
+    """
+    @functools.wraps(test_method)
+    def new_test_method(self):
+        test_generator = test_method(self)
+        root = self.root
+        # Exceptions raised by self.assert...() need to be raised
+        # outside of the after() callback in order for the test
+        # harness to capture them.
+        exception = None
+        def after_callback():
+            nonlocal exception
+            try:
+                next(test_generator)
+            except StopIteration:
+                root.quit()
+            except Exception as exc:
+                exception = exc
+                root.quit()
+            else:
+                # Schedule the Tk mainloop to call this function again,
+                # using a robust method of ensuring that it gets a
+                # chance to process queued events before doing so.
+                # See: https://stackoverflow.com/q/18499082#comment65004099_38817470
+                root.after(1, root.after_idle, after_callback)
+        root.after(0, root.after_idle, after_callback)
+        root.mainloop()
+
+        if exception:
+            raise exception
+
+    return new_test_method