Poetry `add` and `init` can create incompatible settings in pyproject.toml

• 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: Ubuntu Xenial
• Poetry version: 0.12.10
• Link of a Gist with the contents of your pyproject.toml file: https://gist.github.com/nottrobin/0cb0ab3a049891640280fa08f4ca5bcc

Issue

If I try to create a pyproject.toml with Python2 but with Django as a dependency, it creates a pyproject.toml that is incompatible:

$ poetry init --no-interaction --dependency django

This command will guide you through creating your pyproject.toml config.


Using version ^2.1 for django

$ poetry install
Creating virtualenv django-project-py2.7 in /root/.cache/pypoetry/virtualenvs
Installing dependencies from lock file
Warning: The lock file is not up to date with the latest changes in pyproject.toml. You may be getting outdated dependencies. Run update to update them.

[SolverProblemError]
The current project must support the following Python versions: 2.7.12        
Because no versions of django match >=2.1,<2.1.4 || >2.1.4,<3.0               
 and django (2.1.4) requires Python >=3.5, django is forbidden.               
So, because django-project depends on django (^2.1), version solving failed.  

install [--no-dev] [--dry-run] [-E|--extras EXTRAS] [--develop DEVELOP]

This is because these two requirements are fundamentally not quite compatible:

python = "^2.7"
django = "^2.1"

I understand that this might be difficult for Poetry to resolve as it might not have the information that Django 2.1 doesn't work on python < 3.5 at the time it's creating the pyproject.toml. But it's unfortunate that this means that the setup experience for a poetry-based Django project requires such deeper understanding.

Possible solutions could be:

1. poetry init --dependency django would set the version of python in pyproject.toml to ^3.5 (maybe with a warning that your current version of python is lower than that)
2. poetry init --dependency django would error if the current python version was unsupported
3. poetry init --dependency django would select the most recent version of Django supported by the current version of python

I think the nicest experience would be 1. - as you might want to generate a valid pyproject.toml with any version of python, in the full knowledge that you can't actually run the project with that version.