Adding PySide2 fails to resolve version

• I am on the latest Poetry version.
• I have searched the issues of this repo and believe that this is not a duplicate.
• If an exception occurs when executing a command, I executed it again in debug mode (-vvv option).

• OS version and name: Windows 10, Python 3.8.1
• Poetry version: 1.0.2
• Link of a Gist with the contents of your pyproject.toml file: See below

Issue

Adding PySide2 as a dependency seems to trigger a resolver bug. See below. python = "^3.8" seems to clash with PySide2’s limiter < 3.9. The package can be installed without problems in a 3.8.1 venv (pyside2 5.14.0 only works on Python 3.8.1+ on Windows).

~\AppData\Local\Temp
> poetry new ppp
Created package ppp in ppp

~\AppData\Local\Temp
> cd .\ppp\

~\AppData\Local\Temp\ppp
> cat .\pyproject.toml
[tool.poetry]
name = "ppp"
version = "0.1.0"
description = ""
authors = ["..."]

[tool.poetry.dependencies]
python = "^3.8"

[tool.poetry.dev-dependencies]
pytest = "^5.2"

[build-system]
requires = ["poetry>=0.12"]
build-backend = "poetry.masonry.api"

~\AppData\Local\Temp\ppp
> poetry add pyside2=="^5.14"
Creating virtualenv ppp in C:\...\Temp\ppp\.venv

Updating dependencies
Resolving dependencies...

[SolverProblemError]
The current project's Python requirement (^3.8) is not compatible with some of the required packages Python requirement:
  - pyside2 requires Python >=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, <3.9

Because no versions of pyside2 match >5.14,<6.0
 and pyside2 (5.14.0) requires Python >=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, <3.9, pyside2 is forbidden.   So, because ppp depends on pyside2 (^5.14), version solving failed.

[atollk]

Wouldn’t be it useful to improve the error message? Apparently multiple users are or were confused by it.

I think the natural interpretation of the syntax “Python = ^3.8” would be “My code requires features from 3.8, so use any Python version at least 3.8 that works with all packages”. It seems intuitive that Poetry would handle the compatibility and find that Python 3.9 is not an option.

[finswimmer]

Hello,

when poetry resolves the dependency it tries to find a package version that is compatible to all python version, to which the project is compatible. In your case ^3.8 which means every python version >=3.8 and <4.0. pyside2 is not support for python >=3.9. So the dependency resolution fails.

The solution is to restrict the python version your project is compatible, e.g. ~3.8 which means >=3.8,<3.9.

If there exist an package that is compatible for a specific python version, one could also add those constraints to the pyproject.toml. See here.

fin swimmer