Poetry: Dependency (pytype) rejected for project supporting Python 3.6+ even though compatible

[cjolowicz]

Poetry's solver rejects installation of pytype in a project supporting Python 3.6+, even though pytype's Python constraint is compatible with it.

Repro:

1. Add the following pyproject.toml

[tool.poetry]
name = "foobar"
version = "0.1.0"
description = ""
authors = ["User <[email protected]>"]

[tool.poetry.dependencies]
python = "^3.6"

[tool.poetry.dev-dependencies]

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

1. Run poetry add --dev --python="<3.8" pytype

This results in the following error message:

Creating virtualenv poetry-pytype-VZF4hhnk-py3.7 in ~/Library/Caches/pypoetry/virtualenvs
Using version ^2020.2.20 for pytype

Updating dependencies
Resolving dependencies... (0.0s)

[SolverProblemError]
The current project's Python requirement (^3.6) is not compatible with some of the required packages Python requirement:
  - pytype requires Python !=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*,<3.8,>=2.7

Because no versions of pytype match >2020.2.20,<2021.0.0
 and pytype (2020.2.20) requires Python !=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*,<3.8,>=2.7, pytype is forbidden.
So, because poetry-pytype depends on pytype (^2020.2.20), version solving failed.

The same error happens with the command poetry add --dev pytype. (The --python="<3.8" was added because pytype currently does not support Python 3.8.)

[cjolowicz]

pytype 2020.2.20 added the following python_requires declaration:

python_requires = >=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, <3.8

IIUC, Poetry refuses to install pytype because the root project declares compatibility with Python ^3.6. This includes 3.8 and up, which would not be supported by pytype. Poetry does not care that we only install pytype in environments with Python < 3.8, using the --python option to poetry add.

It seems, when determining compatibility of Python version constraints, Poetry does not take into account which Python versions a dependency is installed for.

[cjolowicz]

Adding an explicit >=3.6 works:

$ poetry add --dev --python=">=3.6,<3.8" pytype
Using version ^2020.2.20 for pytype
...

So I guess Poetry did pick up the <3.8 constraint earlier. However, Poetry appears to assume that pytype is also being installed for older Python versions such as 3.4, unsupported by pytype.

Since the root package already requires Python ^3.6, Poetry should be able to work out that pytype cannot possibly be installed in an environment violating the Python version constraint.

[andreas-wolf]

Same bug (?) for new pytype version 2020.7.30 and python 3.8.3:

poetry add --dev pytype
Using version ^2020.7.30 for pytype

Updating dependencies
Resolving dependencies... (0.8s)

[SolverProblemError]
The current project's Python requirement (^3.8) is not compatible with some of the required packages Python requirement:
  - pytype requires Python <3.9,>=3.5

Because no versions of pytype match >2020.7.30,<2021.0.0
 and pytype (2020.7.30) requires Python <3.9,>=3.5, pytype is forbidden.

This seem to work:

poetry add --dev --python=3.8 pytype
Using version ^2020.7.30 for pytype

Updating dependencies
Resolving dependencies... (1.7s)

Writing lock file


Package operations: 0 installs, 1 update, 0 removals

  - Updating cffi (1.14.0 -> 1.14.1)

[finswimmer]

It's not a bug, it's a feature ;) and @cjolowicz is on the right track.

See my comment in https://github.com/python-poetry/poetry/issues/1930#issuecomment-653906544 for more details.

fin swimmer