Pipfile: shell variable assignment from environment variables AND allow defaults

[adriantorrie]

Is your feature request related to a problem? Please describe.

We have wish to have PIP_INDEX_URL default to pypi for our developers on their local machines, but when we run it through our CI we wish to replace the value for PIP_INDEX_URL to point to our private index (corporate).

Describe the solution you'd like

./Pipfile

[[source]]
name = "home"
url = "${PIP_INDEX_URL:-https://pypi.org/simple}"    # Allow shell-like variable assignment
verify_ssl = true

Once a repo is cloned locally by a dev, the isolated python environment can be created by with a simple command:

pipenv install --dev

Within a CI pipeline, the env var PIP_INDEX_URL will be set to the corporate package index and will "just work" as well.

Describe alternatives you've considered

Local developer machine

.env

PIP_INDEX_URL=https://pypi.org/simple

Pipfile

`./Pipfile`
```sh
[[source]]
name = "home"
url = "${PIP_INDEX_URL}"
verify_ssl = true

To install:

pipenv shell

pipenv install --dev

Additional context

Add any other context or screenshots about the feature request here. It may be a good idea to mention that platform and Python version you are on.

---

Please run $ pipenv --support, and paste the results here. Don't put backticks (`) around it! The output already contains Markdown formatting.

If you're on macOS, run the following:

$ pipenv --support | pbcopy

If you're on Windows, run the following:

> pipenv --support | clip

If you're on Linux, run the following:

$ pipenv --support | xclip

[matteius]

@adriantorrie this issue is pretty old at this point, but latest pipenv has support to read from the pip.conf/pip.ini -- perhaps this would resolve this issue for the time being.