When using UV to directly start the REPL, with 3.14.2, AttributeError: module 'readline' has no attribute 'backend'

[BHznJNs]

Bug description:

Not sure is it a bug from uv or cpython.

Python version: 3.14.2
Operating System: Windows 11 25H2

After I installed the virtual environment with uv, I tried to start the REPL with:

uv run python

Then I met this error, here is the complete log:

Python 3.14.2 (main, Jan 27 2026, 23:46:05) [MSC v.1944 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
Failed calling sys.__interactivehook__
Traceback (most recent call last):
  File "<frozen site>", line 542, in register_readline
AttributeError: module 'readline' has no attribute 'backend'
>>>

I also tried the embed version of also 3.14.2, no any error.
I also tried directly launching 3.13.5 and 3.14.2 (both installed with pymanager), also not any error.

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[StanFromIreland]

I also tried directly launching 3.13.5 and 3.14.2 (both installed with pymanager), also no any error.

This points an issue with UV, although I'm not that familiar as to how it interacts with Python.

[tanloong]

You can check if you have installed the pyreadline3 on your machine. The missing backend issue has been there for a while in that package. Uninstall it and see if that works.

[BHznJNs]

Check if you have installed the pyreadline3 on your machine. The missing backend issue has been there for a while in that package. Uninstall it and try REPL again.

It seems like when executing the command uv run python, it will automatically install the pyreadline3 package.

I checked it, there is pyreadline3 package in my venv. Then I uninstalled the package with uv pip uninstall pyreadline3 and checked with uv pip list again.
After I ran uv run python command, the output is here:

=> uv run python
Installed 1 package in 17ms
Python 3.14.2 (main, Jan 27 2026, 23:46:05) [MSC v.1944 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
Failed calling sys.__interactivehook__
Traceback (most recent call last):
  File "<frozen site>", line 542, in register_readline
AttributeError: module 'readline' has no attribute 'backend'
>>>

It shows there was a packaged installed after I run this command. And I checked the package list after this command, the pyreadline3 package appeared.

[tanloong]

It seems there might be a dependency in your pyproject.toml that requires pyreadline3. When sees it's missing, uv run automatically installs it for you. There's an open PR to fix the backend issue in pyreadline3, but it hasn't been merged yet. As a temporary workaround, you can manually add the backend attribute as shown in this PR.

You can find the file to edit with this command:

uv run python -c 'import pyreadline3; from pathlib import Path; print(Path(pyreadline3.__file__).parent.parent / "readline.py")'
# C:\Users\Administrator\Desktop\...\.venv\Lib\site-packages\readline.py

[terryjreedy]

This seems to not be a Python bug.

[BHznJNs]

It seems there might be a dependency in your pyproject.toml that requires pyreadline3. When sees it's missing, uv run automatically installs it for you. There's an open PR to fix the backend issue in pyreadline3, but it hasn't been merged yet. As a temporary workaround, you can manually add the backend attribute as shown in this PR.

Alright, I did not explicitly declare the pyreadline3 dependency, maybe some of my dependency requires it.
That's it, maybe we can close this issue now 😄