pipenv: markers 'platform_python_implementation == "cpython"' don't match your environment when installing gevent

When installing gevent using pipenv, greenlet is not being installed.

{'implementation_name': 'cpython',
 'implementation_version': '3.6.5',
 'os_name': 'posix',
 'platform_machine': 'x86_64',
 'platform_python_implementation': 'CPython',
 'platform_release': '4.16.5-1-ARCH',
 'platform_system': 'Linux',
 'platform_version': '#1 SMP PREEMPT Thu Apr 26 16:53:40 UTC 2018',
 'python_full_version': '3.6.5',
 'python_version': '3.6',
 'sys_platform': 'linux'}

Expected result

I expect installing gevent to also result in greenlet being installed.

Actual result

Instead of having greenlet installed, I get a warning about “Ignoring greenlet: markers ‘platform_python_implementation == “cpython”’ don’t match your environment”.

Steps to replicate

➜  ~/tmp/pipenving echo '[[source]]
url = "https://pypi.org/simple"
verify_ssl = true
name = "pypi"

[packages]
gevent = "*"

[dev-packages]

[requires]
python_version = "2.7"

[pipenv]
allow_prereleases = true' > Pipfile
➜  ~/tmp/pipenving pipenv install
Creating a virtualenv for this project…
Using /home/jeremy/.pyenv/versions/jfly-2.7.12/bin/python2.7 (2.7.12) to create virtualenv…
⠋Running virtualenv with interpreter /home/jeremy/.pyenv/versions/jfly-2.7.12/bin/python2.7
Using real prefix '/home/jeremy/.pyenv/versions/2.7.12'
New python executable in /home/jeremy/.local/share/virtualenvs/pipenving-IYbJmdaL/bin/python2.7
Also creating executable in /home/jeremy/.local/share/virtualenvs/pipenving-IYbJmdaL/bin/python
Installing setuptools, pip, wheel...done.

Virtualenv location: /home/jeremy/.local/share/virtualenvs/pipenving-IYbJmdaL
Pipfile.lock not found, creating…
Locking [dev-packages] dependencies…
Locking [packages] dependencies…
Updated Pipfile.lock (6d8673)!
Installing dependencies from Pipfile.lock (6d8673)…
Ignoring greenlet: markers 'platform_python_implementation == "cpython"' don't match your environment
  🐍   ▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉ 2/2 — 00:00:00
To activate this project's virtualenv, run the following:
 $ pipenv shell
➜  ~/tmp/pipenving pip freeze | grep gevent
You are using pip version 9.0.1, however version 10.0.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
➜  ~/tmp/pipenving pipenv run pip freeze | grep greenlet
➜  ~/tmp/pipenving pipenv run pip freeze | grep gevent
gevent==1.3b1

Note the warning above about platform_python_implementation == “cpython” not matching my environment. I looked into this a bit, and according to PEP508, platform_python_implementation corresponds to platform.python_implementation(), which is equal to “CPython” for me:

➜  ~/tmp/pipenving pipenv run python -c "import platform; print platform.python_implementation()"
CPython

I verified that if I manually change the "markers": "platform_python_implementation == 'cpython'" line in my Pipfile.lock from “cpython” to “CPython”, then pipenv install works happily:

➜  ~/tmp/pipenving sed -i 's/cpython/CPython/' Pipfile.lock 
➜  ~/tmp/pipenving pipenv install                           
Installing dependencies from Pipfile.lock (6d8673)…
  🐍   ▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉▉ 2/2 — 00:00:00
To activate this project's virtualenv, run the following:
 $ pipenv shell
➜  ~/tmp/pipenving pipenv run pip freeze | grep greenlet
greenlet==0.4.13

However, this isn’t a permanent solution for me, as subsequent runs of pipenv lock reset my Pipfile.lock to have a lowercase “cpython”:

➜  ~/tmp/pipenving pipenv lock
Locking [dev-packages] dependencies…
Locking [packages] dependencies…
Updated Pipfile.lock (6d8673)!
➜  ~/tmp/pipenving grep cpython Pipfile.lock 
            "markers": "platform_python_implementation == 'cpython'",

I confirmed that gevent 1.3b1 is correctly capitalizing “CPython” in its setup.py: https://github.com/gevent/gevent/blob/1.3b1/setup.py#L190, so I concluded that pipenv must be lowercasing this marker somewhere. I stepped through pipenv with a debugger to track down where the platform_python_implementation marker is getting lowercased, and I tracked it down to this line:

https://github.com/pypa/pipenv/blob/a16b1d06515b26db929e87905304ba00a9e1f33b/pipenv/patched/piptools/resolver.py#L299

which invokes format_requirement, which lowercases the entire requirement, including the markers:

https://github.com/pypa/pipenv/blob/a16b1d06515b26db929e87905304ba00a9e1f33b/pipenv/patched/piptools/utils.py#L69

It looks like the format_requirement method was written with the intention of markers being specified as an optional second argument, so maybe the right fix is to change our invocation of format_requirement to actually split off the markers and pass them in separately?