ResolutionImpossible if requires-python is empty

[astrojuanlu]

• I have searched the issue tracker and believe that this is not a duplicate.

The pdm init command says "Python requires('*' to allow any)". But if I actually do that, then the resulting pyproject.toml contains the line requires-python = "", which makes resolution fail.

On the other hand, the default requires-python = ">=3.9" does not present any problems.

Steps to reproduce

python3.9 -m venv .venv    
source .venv/bin/activate    
pip install pdm &> /dev/null          
pdm init -n           
sed -i 's;">=3.9";"";g' pyproject.toml          
pdm add -vv "urllib3==1.25.7"    

Actual behavior

$ docker run -it --rm python:3.9 bash
root@97c704248096:/# python3.9 -m venv .venv
root@97c704248096:/# source .venv/bin/activate
(.venv) root@97c704248096:/# pip install pdm &> /dev/null 
(.venv) root@97c704248096:/# pdm init -n
Creating a pyproject.toml for PDM...
Using Python interpreter: /.venv/bin/python3.9 (3.9)
Changes are written to pyproject.toml.
(.venv) root@97c704248096:/# grep requires-python pyproject.toml 
requires-python = ">=3.9"
(.venv) root@97c704248096:/# sed -i 's;">=3.9";"";g' pyproject.toml 
(.venv) root@97c704248096:/# grep requires-python pyproject.toml 
requires-python = ""
(.venv) root@97c704248096:/# pdm add -vv "urllib3==1.25.7"
Adding packages to default dependencies: urllib3
======== Start resolving requirements ========
  urllib3==1.25.7
  python
  Adding requirement urllib3==1.25.7
	Found matching candidates:
	  <Candidate urllib3 1.25.7 from https://pypi.org/simple/urllib3/>
  Adding requirement python
======== Starting round 0 ========
Resolving: new pin python 
======== Ending round 0 ========
======== Starting round 1 ========
  Adding requirement python>=2.7,<4.0,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*(from urllib3 1.25.7)
  Adding requirement python>=2.7,<4.0,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*(from urllib3 1.25.7)
Conflicts detected: 
  python (from project)
  python>=2.7,<4.0,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.* (from <Candidate urllib3 1.25.7 from https://pypi.org/simple/urllib3/>)
  python (from project)
  python>=2.7,<4.0,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.* (from <Candidate urllib3 1.25.7 from https://pypi.org/simple/urllib3/>)
Candidate rejected: python None
🔒 Lock failed
Unable to find a resolution because the following dependencies don't work on all Python versions defined by the project's `requires-python`: 
  python>=2.7,<4.0,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.* (from <Candidate urllib3 1.25.7 from https://pypi.org/simple/urllib3/>)
To fix this, you can change the value of `requires-python` in pyproject.toml.
Traceback (most recent call last):
  File "/.venv/bin/pdm", line 8, in <module>
    sys.exit(main())
  File "/.venv/lib/python3.9/site-packages/pdm/core.py", line 226, in main
    return Core().main(args)
  File "/.venv/lib/python3.9/site-packages/pdm/core.py", line 161, in main
    raise cast(Exception, err).with_traceback(traceback)
  File "/.venv/lib/python3.9/site-packages/pdm/core.py", line 156, in main
    f(options.project, options)
  File "/.venv/lib/python3.9/site-packages/pdm/cli/commands/add.py", line 53, in handle
    actions.do_add(
  File "/.venv/lib/python3.9/site-packages/pdm/cli/actions.py", line 236, in do_add
    resolved = do_lock(project, strategy, tracked_names, reqs, dry_run=dry_run)
  File "/.venv/lib/python3.9/site-packages/pdm/cli/actions.py", line 96, in do_lock
    raise ResolutionImpossible("Unable to find a resolution") from None
resolvelib.resolvers.ResolutionImpossible: Unable to find a resolution
(.venv) root@97c704248096:/# pdm init -n
pyproject.toml already exists, update it now.
Using the last selection, add '-i' to ignore it.
Using Python interpreter: /.venv/bin/python3.9 (3.9)
Changes are written to pyproject.toml.
(.venv) root@97c704248096:/# grep requires-python pyproject.toml 
requires-python = ">=3.9"
(.venv) root@97c704248096:/# pdm add -vv "urllib3==1.25.7"
Adding packages to default dependencies: urllib3
======== Start resolving requirements ========
  urllib3==1.25.7
  python>=3.9
  Adding requirement urllib3==1.25.7
	Found matching candidates:
	  <Candidate urllib3 1.25.7 from https://pypi.org/simple/urllib3/>
  Adding requirement python>=3.9
======== Starting round 0 ========
Resolving: new pin python >=3.9
======== Ending round 0 ========
======== Starting round 1 ========
Resolving: new pin urllib3 1.25.7
======== Ending round 1 ========
======== Starting round 2 ========
======== Resolution Result ========
Stable pins:
   python None
  urllib3 1.25.7
	Found matching candidates:
	  <Candidate urllib3 1.25.7 from https://pypi.org/simple/urllib3/>
🔒 Lock successful
Changes are written to pdm.lock.
Changes are written to pyproject.toml.
Synchronizing working set with lock file: 1 to add, 0 to update, 0 to remove

Install urllib3 1.25.7 successful

🎉 All complete!

Expected behavior

I would have expected requires-python = "" to not cause any issue.

Environment Information

# pdm info && pdm info --env
PDM version:        1.12.8
Python Interpreter: /.venv/bin/python3.9 (3.9)
Project Root:       /
Project Packages:   /__pypackages__/3.9
{
  "implementation_name": "cpython",
  "implementation_version": "3.9.10",
  "os_name": "posix",
  "platform_machine": "x86_64",
  "platform_release": "5.13.0-28-generic",
  "platform_system": "Linux",
  "platform_version": "#31~20.04.1-Ubuntu SMP Wed Jan 19 14:08:10 UTC 2022",
  "python_full_version": "3.9.10",
  "platform_python_implementation": "CPython",
  "python_version": "3.9",
  "sys_platform": "linux"
}

[frostming]

The failure reason is printed in the output:

Unable to find a resolution because the following dependencies don't work on all Python versions defined by the project's `requires-python`: 
  python>=2.7,<4.0,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.* (from <Candidate urllib3 1.25.7 from https://pypi.org/simple/urllib3/>)
To fix this, you can change the value of `requires-python` in pyproject.toml.

Why do you think this is a bug? Close it now.

[astrojuanlu]

The pdm init command says "Python requires('*' to allow any)"

If * leads to "zero Python versions supported" instead of "any Python version supported", then the wording of pdm init is highly misleading IMHO.

[astrojuanlu]

If given *, then pdm init should not produce requires-python metadata at all, if I understood correctly.

[frostming]

If * leads to "zero Python versions supported" instead of "any Python version supported",

No, it does mean any Python version supported. But your dependency urllib3 supports a narrower version range: >=2.7,<4.0,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*. So it can't find a resolution that supports ALL versions that the project promises.

[frostming]

Basically, if the dependency specifies a requires-python, the dependant's requires-python MUST be a subset(narrower) of it, for the good of compatibility. It means "all must work" instead of "any of it works"

[astrojuanlu]

Aaaaaaaah okay, I understand. So requires-python = "" means "this project works with all (any) Python version" which cannot be satisfied if any dependency pins the Python version.

Thanks for the explanation and sorry for the noise.