bpo-38041: Refine IDLE Shell restart lines. (GH-15709)

 Restart lines now always start with '=' and never end with ' ' and fill the width of the window unless that would require ending with ' ', which could be wrapped by itself and possible confusing the user.
(cherry picked from commit 38da805d563422cf1bb9cd9be24c73806840fe30)

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

4 files changed, 44 insertions, 3 deletions

diff --git a/Lib/idlelib/NEWS.txt b/Lib/idlelib/NEWS.txt
index 372c7ef..34cdfbd 100644
--- a/Lib/idlelib/NEWS.txt
+++ b/Lib/idlelib/NEWS.txt
@@ -3,6 +3,10 @@ Released on 2019-09-30?
 ======================================
 
 
+bpo-38401: Shell restart lines now fill the window width, always start
+with '=', and avoid wrapping unnecessarily. The line will still wrap
+if the included file name is long relative to the width.
+
 bpo-37092: Add mousewheel scrolling for IDLE module, path, and stack
 browsers.  Patch by George Zhang.
 
diff --git a/Lib/idlelib/idle_test/test_pyshell.py b/Lib/idlelib/idle_test/test_pyshell.py
index 581444c..4a09667 100644
--- a/Lib/idlelib/idle_test/test_pyshell.py
+++ b/Lib/idlelib/idle_test/test_pyshell.py
@@ -7,6 +7,28 @@ from test.support import requires
 from tkinter import Tk
 
 
+class FunctionTest(unittest.TestCase):
+    # Test stand-alone module level non-gui functions.
+
+    def test_restart_line_wide(self):
+        eq = self.assertEqual
+        for file, mul, extra in (('', 22, ''), ('finame', 21, '=')):
+            width = 60
+            bar = mul * '='
+            with self.subTest(file=file, bar=bar):
+                file = file or 'Shell'
+                line = pyshell.restart_line(width, file)
+                eq(len(line), width)
+                eq(line, f"{bar+extra} RESTART: {file} {bar}")
+
+    def test_restart_line_narrow(self):
+        expect, taglen = "= RESTART: Shell", 16
+        for width in (taglen-1, taglen, taglen+1):
+            with self.subTest(width=width):
+                self.assertEqual(pyshell.restart_line(width, ''), expect)
+        self.assertEqual(pyshell.restart_line(taglen+2, ''), expect+' =')
+
+
 class PyShellFileListTest(unittest.TestCase):
 
     @classmethod
diff --git a/Lib/idlelib/pyshell.py b/Lib/idlelib/pyshell.py
index ea94655..08716a9 100755
--- a/Lib/idlelib/pyshell.py
+++ b/Lib/idlelib/pyshell.py
@@ -387,6 +387,19 @@ class MyRPCClient(rpc.RPCClient):
         "Override the base class - just re-raise EOFError"
         raise EOFError
 
+def restart_line(width, filename):  # See bpo-38141.
+    """Return width long restart line formatted with filename.
+
+    Fill line with balanced '='s, with any extras and at least one at
+    the beginning.  Do not end with a trailing space.
+    """
+    tag = f"= RESTART: {filename or 'Shell'} ="
+    if width >= len(tag):
+        div, mod = divmod((width -len(tag)), 2)
+        return f"{(div+mod)*'='}{tag}{div*'='}"
+    else:
+        return tag[:-2]  # Remove ' ='.
+
 
 class ModifiedInterpreter(InteractiveInterpreter):
 
@@ -491,9 +504,8 @@ class ModifiedInterpreter(InteractiveInterpreter):
         console.stop_readline()
         # annotate restart in shell window and mark it
         console.text.delete("iomark", "end-1c")
-        tag = 'RESTART: ' + (filename if filename else 'Shell')
-        halfbar = ((int(console.width) -len(tag) - 4) // 2) * '='
-        console.write("\n{0} {1} {0}".format(halfbar, tag))
+        console.write('\n')
+        console.write(restart_line(console.width, filename))
         console.text.mark_set("restart", "end-1c")
         console.text.mark_gravity("restart", "left")
         if not filename:
diff --git a/Misc/NEWS.d/next/IDLE/2019-09-05-23-12-13.bpo-38041.nxmGGK.rst b/Misc/NEWS.d/next/IDLE/2019-09-05-23-12-13.bpo-38041.nxmGGK.rst
new file mode 100644
index 0000000..0aa254e
--- /dev/null
+++ b/Misc/NEWS.d/next/IDLE/2019-09-05-23-12-13.bpo-38041.nxmGGK.rst
@@ -0,0 +1,3 @@
+Shell restart lines now fill the window width, always start with '=',
+and avoid wrapping unnecessarily. The line will still wrap if the
+included file name is long relative to the width.