WIP debugger: add 'show_output' option for single key press on output…

… screen

Introduce the 'show_output' config option, pass its value from the
debugger's .show_output() method to the StoppedScreen class. Default
to the 'python_input' selection for backwards compatibility. Support
'single_key' as an alternative, which accepts additional optional
keywords: 'silent' to not prompt for key presses, and several sets
of terminating key presses like 'o_space_enter', 'enter_only', or
'any_key' to reduce behavioural changes from to earlier versions.

  show_output = python_input
  show_output = single_key
  show_output = single_key silent o_space_enter

In the absence of terminating key press specs, any key press leaves
the output screen. The implementation uses Python select(3) which may
not work with stdin on Windows. The StoppedScreen implementation lends
itself to more keywords and other ways of getting user input if desired.

TODO
- Config file handling is incomplete. Implements the keyword and its
  default value, but lacks documentation and UI controls.
- Move NonBlockingConsole to a better location, common support or
  platform dependent?
- Drop diagnostics.
- Is it confusing to have too many configuration options? Just have
  "single key, any key" as one boolean choice while the default is
  the input(3) invocation that is portable to all platforms?
- Maybe adjust Python style where required.

pudb/debugger.py

@@ -27,7 +27,10 @@
 import bdb
 import gc
 import os
+import select
 import sys
+import termios
+import tty
 from collections import deque
 from functools import partial
 from itertools import count
@@ -762,17 +765,78 @@ def get_lines(self, debugger_ui):
 
 # }}}
 
+class NonBlockingConsole(object):
+
+    def __init__(self, timeout = 0):
+        self.timeout = timeout
+
+    def __enter__(self):
+        self.old_settings = termios.tcgetattr(sys.stdin)
+        tty.setcbreak(sys.stdin.fileno())
+        return self
+
+    def __exit__(self, type, value, traceback):
+        termios.tcsetattr(sys.stdin, termios.TCSADRAIN, self.old_settings)
+
+    def get_data(self):
+        rset, wset, eset = select.select([ sys.stdin, ], [], [], self.timeout)
+        if sys.stdin in rset:
+            return sys.stdin.read(1)
+        return None
 
 class StoppedScreen:
-    def __init__(self, screen):
+    def __init__(self, screen, show_output = None):
+        # print("config option: {}".format(show_output), flush = True, file = sys.stderr)
         self.screen = screen
+        self.show_output = show_output
+        if "single_key" in show_output:
+            show_output = self.show_output.replace("single_key", "")
+            self.with_prompt = "silent" not in show_output
+            show_output = show_output.replace("silent", "")
+            show_output = show_output.strip()
+            # TODO Alternatively pick key names from remaining bare words?
+            # Could be perceived as more intuitive by users.
+            self.term_keys = {
+                "o_space_enter": "o \n",
+                "any_key": None,
+                "enter_only": "\n",
+            }.get(show_output, None)
+        else:
+            self.show_output = "python_input"
 
     def __enter__(self):
         self.screen.stop()
+        return self
 
     def __exit__(self, exc_type, exc_value, exc_traceback):
         self.screen.start()
 
+    def press_key_to_return(self):
+        if "single_key" in self.show_output:
+            # Locally modified behaviour.
+            # BEWARE! The urwid screen is stopped when this method
+            # executes. Curses calls are not available, neither are
+            # urwid widgets nor key event handlers. This is why the
+            # default activity is to invoke Python's input(3).
+            # That's why something non-portable is done, fiddling
+            # with terminal settings. Which get restored afterwards.
+            if self.with_prompt:
+                print("Hit a key to return ", end = "", flush = True)
+            with NonBlockingConsole(timeout = None) as nbc:
+                while True:
+                    key = nbc.get_data()
+                    if not self.term_keys:
+                        break
+                    if key in self.term_keys:
+                        break
+            if self.with_prompt:
+                print("")
+            return
+        # The default behaviour, requires pressing ENTER.
+        # Also the fall through for unmigrated configurations.
+        if "python_input" in self.show_output or not self.show_output:
+            input("Hit Enter to return:")
+            return
 
 class DebuggerUI(FrameVarInfoKeeper):
     # {{{ constructor
@@ -2082,8 +2146,8 @@ def shrink_sidebar(w, size, key):
         # {{{ top-level listeners
 
         def show_output(w, size, key):
-            with StoppedScreen(self.screen):
-                input("Hit Enter to return:")
+            with StoppedScreen(self.screen, CONFIG["show_output"]) as s:
+                s.press_key_to_return()
 
         def reload_breakpoints_and_redisplay():
             reload_breakpoints()

pudb/settings.py

@@ -113,6 +113,7 @@ def load_config():
     conf_dict.setdefault("display", "auto")
 
     conf_dict.setdefault("prompt_on_quit", "True")
+    conf_dict.setdefault("show_output", "python_input")
 
     conf_dict.setdefault("hide_cmdline_win", "False")
 
@@ -220,6 +221,8 @@ def _update_config(check_box, new_state, option_newvalue):
             conf_dict.update(new_conf_dict)
             _update_prompt_on_quit()
 
+# XXX need to handle "show_output" here?
+
         elif option == "hide_cmdline_win":
             new_conf_dict["hide_cmdline_win"] = not check_box.get_state()
             conf_dict.update(new_conf_dict)
@@ -267,6 +270,8 @@ def _update_config(check_box, new_state, option_newvalue):
             bool(conf_dict["prompt_on_quit"]), on_state_change=_update_config,
                 user_data=("prompt_on_quit", None))
 
+# XXX need to handle "show_output" here?
+
     hide_cmdline_win = urwid.CheckBox("Hide command line"
             f"({conf_dict['hotkeys_toggle_cmdline_focus']}) window "
                                       "when not in use",