[BUG] Should fail hard when installing packages that want 2to3

[plumdog]

setuptools version

setuptools==58.0.0

Python version

Python 3.9

OS

Linux

Additional environment information

No response

Description

Tried to install a package that uses 2to3 (specifically https://pypi.org/project/demjson/), got a message from pip like:

Successfully installed demjson-2.2.3

But really it wasn't successful, and then get confusing SyntaxError exceptions.

Expected behavior

I have now worked out that this is because of the change to remove 2to3 from setuptools, which I think is a perfectly reasonable change, and because my setuptools version wasn't pinned. But setuptools is not doing something the package is asking for, and the result is going to be something that does not run (though I guess there are caveats here).

I would have expected to get an error like "This module wants to use 2to3, but this is not supported [link to github issue about removing 2to3 support]". At least, that would certainly have saved me time.

How to Reproduce

Install demjson==2.2.3 with setuptools==58.0.0. Attempt to import and get a syntax error.

Output

$ pip install -U setuptools==58.0.0 && pip install demjson==2.2.3 && python -c 'import demjson'
Collecting setuptools==58.0.0
  Using cached setuptools-58.0.0-py3-none-any.whl (816 kB)
Installing collected packages: setuptools
  Attempting uninstall: setuptools
    Found existing installation: setuptools 56.0.0
    Uninstalling setuptools-56.0.0:
      Successfully uninstalled setuptools-56.0.0
Successfully installed setuptools-58.0.0
Collecting demjson==2.2.3
  Using cached demjson-2.2.3.tar.gz (131 kB)
Using legacy 'setup.py install' for demjson, since package 'wheel' is not installed.
Installing collected packages: demjson
    Running setup.py install for demjson ... done
Successfully installed demjson-2.2.3
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/user/example/venv/lib/python3.9/site-packages/demjson.py", line 645
    class json_int( (1L).__class__ ):    # Have to specify base this way to satisfy 2to3
                      ^
SyntaxError: invalid syntax

Code of Conduct

• I agree to follow the PSF Code of Conduct

[jaraco]

Good suggestion. I think I agree.

[arpheno]

I think this broke a lot of builds unintentionally.

[ikapelyukhin]

I think this broke a lot of builds unintentionally.

It sure did. 😬

[plumdog]

My intention when raising this issue was to make it so builds fail, rather than getting runtime errors because 2to3 wasn't applied to a dependency that needs it. That is, this behaviour breaks builds intentionally where the setuptools version and the dependencies are incompatible.

[mattclay]

@plumdog Unfortunately the implementation in #2770 also breaks builds which explicitly disable the use_2to3 feature, which would otherwise install correctly.

[plumdog]

@mattclay Oh. 😬

[ajnelson-nist]

@jaraco , I apologize, I just found I'd forgotten to reply.

Your reasoning makes sense, and I won't push the issue further. What I hadn't appreciated fully is that 2to3-related breakage was supposed to be introduced with this major release. Thank you for the kind discussion.