IDLE -- Restrict shell prompt manipulaton to the shell. (#4143)

Editor and output windows only see an empty last prompt line.
This simplifies the code and fixes a minor bug when newline is inserted.
Sys.ps1, if present, is read on Shell start-up, but is not set or changed.

Lib/idlelib/pyshell.py

@@ -857,15 +857,17 @@ class PyShell(OutputWindow):
             fixwordbreaks(root)
             root.withdraw()
             flist = PyShellFileList(root)
-        #
+
         OutputWindow.__init__(self, flist, None, None)
-        #
-##        self.config(usetabs=1, indentwidth=8, context_use_ps1=1)
+
         self.usetabs = True
         # indentwidth must be 8 when using tabs.  See note in EditorWindow:
         self.indentwidth = 8
-        self.context_use_ps1 = True
-        #
+
+        self.sys_ps1 = sys.ps1 if hasattr(sys, 'ps1') else '>>> '
+        self.prompt_last_line = self.sys_ps1.split('\n')[-1]
+        self.prompt = self.sys_ps1  # Changes when debug active
+
         text = self.text
         text.configure(wrap="char")
         text.bind("<<newline-and-indent>>", self.enter_callback)
@@ -878,7 +880,7 @@ class PyShell(OutputWindow):
         if use_subprocess:
             text.bind("<<view-restart>>", self.view_restart_mark)
             text.bind("<<restart-shell>>", self.restart_shell)
-        #
+
         self.save_stdout = sys.stdout
         self.save_stderr = sys.stderr
         self.save_stdin = sys.stdin
@@ -951,7 +953,7 @@ class PyShell(OutputWindow):
                 debugger_r.close_remote_debugger(self.interp.rpcclt)
             self.resetoutput()
             self.console.write("[DEBUG OFF]\n")
-            sys.ps1 = ">>> "
+            self.prompt = self.sys_ps1
             self.showprompt()
         self.set_debugger_indicator()
 
@@ -963,7 +965,7 @@ class PyShell(OutputWindow):
             dbg_gui = debugger.Debugger(self)
         self.interp.setdebugger(dbg_gui)
         dbg_gui.load_breakpoints()
-        sys.ps1 = "[DEBUG ON]\n>>> "
+        self.prompt = "[DEBUG ON]\n" + self.sys_ps1
         self.showprompt()
         self.set_debugger_indicator()
 
@@ -1248,11 +1250,7 @@ class PyShell(OutputWindow):
 
     def showprompt(self):
         self.resetoutput()
-        try:
-            s = str(sys.ps1)
-        except:
-            s = ""
-        self.console.write(s)
+        self.console.write(self.prompt)
         self.text.mark_set("insert", "end-1c")
         self.set_line_and_column()
         self.io.reset_undo()