GH-130328: pasting in new REPL is slow on Windows #132884

chris-eibl · 2025-04-24T17:22:28Z

The reason why pasting is so slow on Windows (especially in the legacy console case where virtual terminal mode - and thus bracketed paste - is disabled):

cpython/Lib/_pyrepl/reader.py

Lines 702 to 706 in 9f5994b

    
           while True: 
        
               # We use the same timeout as in readline.c: 100ms 
        
               self.run_hooks() 
        
               self.console.wait(100) 
        
               event = self.console.get_event(block=False)

and

cpython/Lib/_pyrepl/windows_console.py

Lines 523 to 532 in 9f5994b

    
           def wait(self, timeout: float | None) -> bool: 
        
               """Wait for an event.""" 
        
               # Poor man's Windows select loop 
        
               start_time = time.time() 
        
               while True: 
        
                   if msvcrt.kbhit(): # type: ignore[attr-defined] 
        
                       return True 
        
                   if timeout and time.time() - start_time > timeout / 1000: 
        
                       return False 
        
                   time.sleep(0.01)

It is interesting, that msvcrt.kbhit() returns True in case of pasting, but if that weren't the case, we'd hit the 100ms timeout and would be even way slower. However, msvcrt.kbhit() is very slow in case of the legacy console, and it is called at least twice as often, because we get a key up and a key down event - but only a key down event in the virtual terminal case. If the pasted input contains upper case letters, we additionally get a "shift key pressed" event - so in the worst case, msvcrt.kbhit() gets called 3 times more often (and each call is slower).

Output of python.bat -m cProfile -m _pyrepl when pasting the "test value" given in the OP for a legacy console:

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
     50/1    0.010    0.000   17.202   17.202 {built-in method builtins.exec}
        1    0.000    0.000   17.202   17.202 <frozen runpy>:201(run_module)
        1    0.000    0.000   17.199   17.199 <frozen runpy>:65(_run_code)
        1    0.000    0.000   17.199   17.199 __main__.py:1(<module>)
        1    0.000    0.000   17.112   17.112 main.py:24(interactive_console)
        1    0.000    0.000   17.112   17.112 simple_interact.py:99(run_multiline_interactive_console)
        5    0.000    0.000   17.111    3.422 readline.py:375(multiline_input)
        5    0.004    0.001   17.111    3.422 reader.py:741(readline)
     3090    0.036    0.000   17.103    0.006 reader.py:694(handle1)
     6210    0.013    0.000   13.714    0.002 windows_console.py:523(wait)
     6236   13.433    0.002   13.433    0.002 {built-in method msvcrt.kbhit}

Here the relevant line in case of virtual terminal:

  3270    0.554    0.000    0.554    0.000 {built-in method msvcrt.kbhit}

The fix is to use WaitForSingleObject

    def wait(self, timeout: float | None) -> bool:
        """Wait for an event."""
        ret = WaitForSingleObject(InHandle, int(timeout))
        if ret == WAIT_FAILED:
            raise WinError(ctypes.get_last_error())

which speeds up especially the legacy console (times in seconds):

	legacy terminal	virtual terminal
before	15.6	0.89
after	1.8	0.26

Note, see also #132440 (comment):

legacy terminal: manually start cmd.exe. Timings gotten by pasting

import time
t1 = time.time()
<"test value" given in the OP>
print(time.time() - t1)

virtual terminal: power shell via Windows terminal. Timings gotton by temporarily patching commands.py:

import time
class enable_bracketed_paste(Command):
    def do(self) -> None:
        self.reader.bp_begin = time.perf_counter()
        self.reader.paste_mode = True
        self.reader.in_bracketed_paste = True

class disable_bracketed_paste(Command):
    def do(self) -> None:
        print("bracketed_paste took %5.2f s" % (time.perf_counter() - self.reader.bp_begin, ))
        self.reader.paste_mode = False
        self.reader.in_bracketed_paste = False
        self.reader.dirty = True

Issue: Pasting in new REPL is slow #130328

sergey-miryanov · 2025-04-24T17:29:10Z

Good analysis and speedup! 👍

Lib/_pyrepl/windows_console.py

chris-eibl · 2025-04-24T18:09:10Z

Lib/_pyrepl/windows_console.py

-            if not events.value:
-                return None
+        if not block and not self.wait(timeout=0):
+            return None


Now we are nicely in sync with unix_console.py

cpython/Lib/_pyrepl/unix_console.py

Lines 400 to 401 in e1c09ff

if not block and not self.wait(timeout=0):

return None

Lib/_pyrepl/windows_console.py

Fidget-Spinner · 2025-04-24T21:42:37Z

We generally don't backport perf fixes unless they lead to a denial-of-service problem (so usually like an exponential-time algorithm), or result in a serious bug. So I'm removing the backport label.

Fidget-Spinner · 2025-04-24T22:04:40Z

On second thought, "slow" is perhaps an understatement. So once this is merged, let's ping Thomas to ask for his permission to backport to 3.13

chris-eibl · 2025-04-24T22:19:09Z

Yeah, in case of the legacy console pasting of the 30 + 3 lines given by the OP takes 15 seconds, and this brings it down to 1.8. Most people will hopefully use a virtual terminal, where the situation is much less severe. After all, the new REPL can be disabled using https://docs.python.org/3/using/cmdline.html#envvar-PYTHON_BASIC_REPL.

3 more lines added by me for easier benchmarking #130328 (comment)

import time
t1 = time.time()

"test value" given in the OP

"""1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890"""

print(time.time() - t1)

pablogsal · 2025-04-25T00:57:24Z

If @ambv has not checked this this in a couple of days ping me and I will do a pass and land it. Thanks a lot for looking into this @chris-eibl ❤️

in class Reader

This reverts commit bba26c9.

ambv · 2025-04-29T16:08:02Z

Thanks for the fix, Chris!

Fidget-Spinner · 2025-04-29T16:12:34Z

@Yhg1s seeking permission to backport this to 3.13.

According to Chris, it takes 15 seconds to paste 58 lines in the new repl in 3.13. Do you think that's slow enough to be worth backporting?

chris-eibl added 2 commits April 24, 2025 18:02

fix WindowsConsole.wait()

0a38c2e

blurb it

05d26ff

chris-eibl requested review from pablogsal, lysnikolaou and ambv as code owners April 24, 2025 17:22

bedevere-app bot added the awaiting review label Apr 24, 2025

bedevere-app bot mentioned this pull request Apr 24, 2025

Pasting in new REPL is slow #130328

Open

chris-eibl added the needs backport to 3.13 bugs and security fixes label Apr 24, 2025

fix timeout=None

cad6bfb

chris-eibl commented Apr 24, 2025

View reviewed changes

Lib/_pyrepl/windows_console.py Show resolved Hide resolved

chris-eibl added 2 commits April 24, 2025 19:53

fix mypy for ctypes.get_last_error

3d1da66

fix return value of wait() and use it in _read_input()

4098e3d

chris-eibl commented Apr 24, 2025

View reviewed changes

tomasr8 reviewed Apr 24, 2025

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Lib/_pyrepl/windows_console.py Show resolved Hide resolved

chris-eibl mentioned this pull request Apr 24, 2025

GH-130328: further speedup of pasting in new REPL on Windows by reading in chunks #132889

Open

chris-eibl added OS-windows topic-repl Related to the interactive shell labels Apr 24, 2025

Fidget-Spinner removed the needs backport to 3.13 bugs and security fixes label Apr 24, 2025

chris-eibl mentioned this pull request Apr 25, 2025

GH-132439: REPL on Windows swallows characters entered via AltGr #132440

Open

chris-eibl added 2 commits April 28, 2025 16:54

use self.paste_mode instead of self.in_bracketed_paste

bba26c9

in class Reader

Revert "use self.paste_mode instead of self.in_bracketed_paste"

3c0cae9

This reverts commit bba26c9.

ambv merged commit acb222c into python:main Apr 29, 2025
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GH-130328: pasting in new REPL is slow on Windows #132884

GH-130328: pasting in new REPL is slow on Windows #132884

chris-eibl commented Apr 24, 2025 •

edited by bedevere-app bot

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sergey-miryanov commented Apr 24, 2025

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pablogsal commented Apr 25, 2025

ambv commented Apr 29, 2025

Fidget-Spinner commented Apr 29, 2025

	while True:
	# We use the same timeout as in readline.c: 100ms
	self.run_hooks()
	self.console.wait(100)
	event = self.console.get_event(block=False)

	def wait(self, timeout: float \| None) -> bool:
	"""Wait for an event."""
	# Poor man's Windows select loop
	start_time = time.time()
	while True:
	if msvcrt.kbhit(): # type: ignore[attr-defined]
	return True
	if timeout and time.time() - start_time > timeout / 1000:
	return False
	time.sleep(0.01)

GH-130328: pasting in new REPL is slow on Windows #132884

GH-130328: pasting in new REPL is slow on Windows #132884

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