debugger: leave output screen with single key press

Introduce support code to get single key presses from a console. Support
Windows (msvcrt) and POSIX supporting Unix platforms (select, termios).
Fall back to Python's input() routine for Webassembly and Emscripten.

Avoid the accumulation of repeated prompts on platforms that support
single key presses. Only the backwards compatible fallback on minor
platforms involves a prompt, and users can tell when they face that
situation.

This change increases usability, a single key press leaves the output
screen after it was entered by pressing 'o'. Does not require a config
item, platform detection is automatic. The approach uses Python core
libraries, no external dependencies are involved. The implementation
is considered maintainable, lends itself to future extension for more
platforms as the need gets identified.

.pylintrc-local.yml

@@ -4,3 +4,4 @@
   - IPython
   - py.test
   - pytest
+  - msvcrt

pudb/debugger.py

@@ -35,7 +35,7 @@
 
 import urwid
 
-from pudb.lowlevel import decode_lines, ui_log
+from pudb.lowlevel import ConsoleSingleKeyReader, decode_lines, ui_log
 from pudb.settings import get_save_config_path, load_config, save_config
 
 
@@ -769,10 +769,15 @@ def __init__(self, screen):
 
     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):
+        with ConsoleSingleKeyReader() as key_reader:
+            key_reader.get_single_key()
+
 
 class DebuggerUI(FrameVarInfoKeeper):
     # {{{ constructor
@@ -2082,8 +2087,8 @@ 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) as s:
+                s.press_key_to_return()
 
         def reload_breakpoints_and_redisplay():
             reload_breakpoints()

pudb/lowlevel.py

@@ -1,6 +1,7 @@
 __copyright__ = """
 Copyright (C) 2009-2017 Andreas Kloeckner
 Copyright (C) 2014-2017 Aaron Meurer
+Copyright (C) 2024 Gerhard Sittig
 """
 
 __license__ = """
@@ -27,6 +28,7 @@
 import logging
 import sys
 from datetime import datetime
+from enum import Enum, auto
 
 
 logfile = [None]
@@ -283,4 +285,81 @@ def decode_lines(lines):
 
 # }}}
 
+
+# {{{ get single key press from console outside of curses
+
+class KeyReadImpl(Enum):
+    INPUT = auto()
+    GETCH = auto()
+    SELECT = auto()
+
+
+_keyread_impl = KeyReadImpl.INPUT
+if sys.platform in ("emscripten", "wasi"):
+    pass
+elif sys.platform in ("win32",):
+    _keyread_impl = KeyReadImpl.GETCH
+else:
+    _keyread_impl = KeyReadImpl.SELECT
+
+
+class ConsoleSingleKeyReader:
+    """
+    Get a single key press from a terminal without a prompt.
+
+    Eliminates the necessity to press ENTER before other input also
+    becomes available. Avoids the accumulation of prompts on the screen
+    as was the case with Python's input() call. Is used in situations
+    where urwid is disabled and curses calls are not available.
+
+    Supports major desktop platforms with special cases (msvcrt getch(),
+    termios and select). Transparently falls back to Python's input()
+    method. Call sites remain simple and straight forward.
+    """
+
+    def __enter__(self):
+        if _keyread_impl == KeyReadImpl.SELECT:
+            import termios
+            import tty
+            self.prev_settings = termios.tcgetattr(sys.stdin)
+            tty.setcbreak(sys.stdin.fileno())
+        return self
+
+    def __exit__(self, type, value, traceback):
+        if _keyread_impl == KeyReadImpl.SELECT:
+            import termios
+            termios.tcsetattr(sys.stdin, termios.TCSADRAIN, self.prev_settings)
+
+    def get_single_key(self):
+        if _keyread_impl == KeyReadImpl.GETCH:
+            import msvcrt
+            # https://docs.python.org/3/library/msvcrt.html#msvcrt.getch
+            # Most keys are returned in the first getch() call. Some
+            # special keys (function keys, cursor, keypad) require
+            # another call when the first returned '\0' or '\xe0'.
+            c = msvcrt.getch()
+            if c in ("\x00", "\xe0"):
+                c = msvcrt.getch()
+            return c
+
+        elif _keyread_impl == KeyReadImpl.SELECT:
+            import select
+            rset, _, _ = select.select([sys.stdin], [], [], None)
+            assert sys.stdin in rset
+            return sys.stdin.read(1)
+
+        # Strictly speaking putting the fallback here which requires
+        # pressing ENTER is not correct, this is the "non buffered"
+        # console support code. But it simplifies call sites. And is
+        # easy to tell by users because a prompt is provided. This is
+        # the most portable approach, and backwards compatible with
+        # earlier PuDB releases. It's a most appropriate default for
+        # otherwise unsupported platforms. Or when users choose to
+        # not accept single key presses, or keys other than ENTER.
+        else:
+            input("Hit Enter to return:")
+            return None
+
+# }}}
+
 # vim: foldmethod=marker