Poetry: Poetry unable to add pex to Python3 project

[taranjlu]

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

• OS version and name: Pop_OS! 19.10
• Poetry version: 1.0.5
• Link of a Gist with the contents of your pyproject.toml file: https://gist.github.com/taranjlu/bab07ec1711e26d5394c5cb47673bf51

Issue

Trying to add pex as a dev (or regular) dependency fails with a SolverProblemError:
```lang=sh
poetry new --src poetry_bug
cd poetry_bug
poetry add --dev pex

```lang=sh
The currently activated Python version 2.7.17 is not supported by the project (^3.4).
Trying to find and use a compatible version. 
Trying python3
Using python3 (3.7.7)
Virtualenv poetry-bug-3oKZJDu--py3.7 already exists.
Using virtualenv: /home/----/.cache/pypoetry/virtualenvs/poetry-bug-3oKZJDu--py3.7
PyPI: 97 packages found for pex *
Using version ^2.1.8 for pex

Updating dependencies
Resolving dependencies...
   1: fact: poetry-bug is 0.1.0
   1: derived: poetry-bug
   1: fact: poetry-bug depends on pytest (^4.6)
   1: fact: poetry-bug depends on pex (^2.1.8)
   1: fact: poetry-bug depends on pytest (^4.6)
   1: fact: poetry-bug depends on pex (^2.1.8)
   1: selecting poetry-bug (0.1.0)
   1: derived: pex (^2.1.8)
   1: derived: pytest (^4.6)
PyPI: 1 packages found for pex >=2.1.8,<3.0.0
   1: fact: pex (2.1.8) requires Python >=2.7,<3.9,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*
   1: derived: not pex (2.1.8)
   1: fact: no versions of pex match >2.1.8,<3.0.0
   1: conflict: no versions of pex match >2.1.8,<3.0.0
   1: ! pex (>2.1.8,<3.0.0) is partially satisfied by not pex (2.1.8)
   1: ! which is caused by "pex (2.1.8) requires Python >=2.7,<3.9,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*"
   1: ! thus: pex is forbidden
   1: ! pex (>=2.1.8,<3.0.0) is satisfied by pex (^2.1.8)
   1: ! which is caused by "poetry-bug depends on pex (^2.1.8)"
   1: ! thus: version solving failed
   1: Version solving took 0.015 seconds.
   1: Tried 1 solutions.

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

Because no versions of pex match >2.1.8,<3.0.0
 and pex (2.1.8) requires Python >=2.7,<3.9,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*, pex is forbidden.
So, because poetry-bug depends on pex (^2.1.8), version solving failed.

Traceback (most recent call last):
  File "/home/----/.poetry/lib/poetry/_vendor/py2.7/clikit/console_application.py", line 131, in run
    status_code = command.handle(parsed_args, io)
  File "/home/----/.poetry/lib/poetry/_vendor/py2.7/clikit/api/command/command.py", line 120, in handle
    status_code = self._do_handle(args, io)
  File "/home/----/.poetry/lib/poetry/_vendor/py2.7/clikit/api/command/command.py", line 171, in _do_handle
    return getattr(handler, handler_method)(args, io, self)
  File "/home/----/.poetry/lib/poetry/_vendor/py2.7/cleo/commands/command.py", line 92, in wrap_handle
    return self.handle()
  File "/home/----/.poetry/lib/poetry/console/commands/add.py", line 149, in handle
    status = installer.run()
  File "/home/----/.poetry/lib/poetry/installation/installer.py", line 74, in run
    self._do_install(local_repo)
  File "/home/----/.poetry/lib/poetry/installation/installer.py", line 161, in _do_install
    ops = solver.solve(use_latest=self._whitelist)
  File "/home/----/.poetry/lib/poetry/puzzle/solver.py", line 36, in solve
    packages, depths = self._solve(use_latest=use_latest)
  File "/home/----/.poetry/lib/poetry/puzzle/solver.py", line 190, in _solve
    raise SolverProblemError(e)

Seems to work correctly when using Python2. Activating the virtualenv and installing directly via pip also works just fine.

Also tested this on macOS.

[finswimmer]

Hello @taranjlu ,

poetry tries to find a major version of a package that is available for all python version that can be used within a project. In your pyproject.toml you have defined ^3.4 for python. This means all python versions >=3.4 but <4.

pex has defined that it is not compatible with python 3.4 and the version must be <3.9. So that's in conflict with your project requirements.

Solution: Set your python requirement to >3.4,<3.9

TL,DR: poetry is only satisfied if it can resolve dependencies for python version specified in the pyproject.toml and not only for the currently used one.

fin swimmer

[taranjlu]

Ah, okay, I see. Thank you for clearing that up. I had assumed that it would look at the various dependencies and requirements to determine if any valid configuration exists and then use that.