Python 3.12 runs much slower than Python 3.11

[cvzi]

Environment data

• debugpy version: 1.8.0
• OS and version: Windows 10
• Python version: 3.12 64bit
• Using VS Code

Actual behavior

Importing the emoji module (v2.10.0) with debugging enabled takes a long time with Python 3.12 compared to Python 3.11.

When running from VS Code with the default debug configuration via F5, the import takes 4 minutes on Python 3.12 but only 50ms on Python 3.11

The emoji module contains a large Python file, here. I suspect the slow debugging has something to do with this file.

Expected behavior

Similar run times in Python 3.11 and Python 3.12

Steps to reproduce:

1. Install emoji module: pip install emoji==2.10.0
2. Create a test file in VS Code that imports the emoji module:

   import time
   start = time.time()
   
   import emoji
   
   end = time.time()
   print("import time", end - start)

3. Set VS Code to Python 3.12
4. Run in VS Code by pressing F5

[int19h]

I suspect it's the large dict literal specifically that is the issue there. It sounds like in 3.12 it is reporting a debugger "line" event (which the debugger then has to process to check for breakpoints etc) for every subexpression in the literal, whereas before it was treated as a single atomic statement?

If so, then this is to be expected - settrace-based Python debugging is inherently slow (~20x overhead per line of pure Python code, such as here). However, we're working on debugpy v2 utilizing the new sys.monitoring API in Python 3.12 for near-zero-overhead debugging; that should address this issue. And I think we should add this import as one of the test cases for that.

[lsmith77]

@int19h it sounds like this will take a bit until it is ready. is there any quick fix/work-around for this? it also appears to affect spacy.

[bersbersbers]

I think the original issue has been fixed by emoji v2.10.1. For anyone working on this, here's another example:

bug.py:

import timeit
print(timeit.timeit("import colorcet", number=1))

Then run:

pip install colorcet debugpy
python bug.py
python -m debugpy --listen 5678 bug.py

For me, this prints

0.23384420000002137
0.01s - Debugger warning: It seems that frozen modules are being used, which may
0.00s - make the debugger miss breakpoints. Please pass -Xfrozen_modules=off
0.00s - to python to disable frozen modules.
0.00s - Note: Debugging will proceed. Set PYDEVD_DISABLE_FILE_VALIDATION=1 to disable this validation.
15.49023370000009

Python 3.11 takes ~0.03s vs. 0.05s, so even without the debugger, Python 3.11 seems to be an order of magnitude faster. But the difference is much more striking with the debugger.

Implementation is at https://github.com/holoviz/colorcet/blob/main/colorcet/__init__.py. I also opened holoviz/colorcet#121.

[cvzi]

Yes it was "solved" in emoji module by compressing the dict literal into a single line, previously the dict was spread over ~87k lines

[cvzi]

FYI for anyone considering this solution, unfortunately compressing the dict into a single line has created a new problem for us, it causes coverage report on Python 3.12 to use excessive amounts of memory (in our case more than 20 GB).

[bersbersbers]

causes coverage report on Python 3.12 to use excessive amounts of memory

For the record, this issue (nedbat/coveragepy#1785) should be gone with Python 3.12.4 thanks to python/cpython#119118, so the practicality of one-line workarounds should be improved. That is relevant since the original issue continues to exist in Python 3.12.4 (long runtime importing large dicts with debugger attached)

[Denelbe]

I have a similar problem with module python-opcua.
The simple following script in vscode debugger runs in 2.7 seconds using Python 3.11.2 but 68 seconds with Python 3.12.5.

from opcua import Server 
server = Server()

OS Linux (Debian 12)
VsCode 1.92.2
Python Debugger 2024.10.0

[snmishra]

I've moved to pydevd. debugpy is unusable in Python 3.12, and I didn't want to go back to Python 3.11

[rchiodo]

Debugpy actually uses pydevd under the covers, but hasn't yet updated to pydevd's latest which has support for 3.12.

[laipz8200]

I am currently debugging the repository langgenius/dify and am encountering a similar issue with the latest version, where the project takes an unusually long time to start (approximately 5 minutes) on my M1 Pro machine.

I would like to understand the underlying cause of this delay and request guidance on how to resolve it.

Environment Details:

• Python Version: 3.12.5
• Operating System: macOS 15.0
• Chip: Apple M1 Pro

Thank you for your assistance!

Feel free to let me know if you would like any additional information.