new 'pre' syntax in Pipfile

[gsemet]

Placeholder ticket for the proposition we talk with Kenneth.

Add the following syntax in the Pipfile:

...
mypkg = { version = "*", pre="true"}
...

This would allow to allow prerelease resolution for one package only.

Change the pipenv install mypkg --pre behavior. It will set the setting described above only for this very package. Today it turns on a general 'pre' setting for the whole Pipfile, leading other packages to also resolve to prerelease.

Change the pipenv update and pipenv lock behavior. They should take into account the presence of the pre=true setting for some package.

Here are some impact I think might happen

• if a version of package is describe at >=1.0 in one dependency, and another package define the pre=true while a new version 2.0.0.b1 is available on pypi, pipenv install should take it
• I don't see any syntax to turn the general pre setting in the Pipfile. Maybe we can document users need to do a pipenv update --pre or pipenv lock --pre to turn the general 'pre' settings on.

[gsemet]

I am assigning myself to this task, but I might take some time to enter into the code

[techalchemy]

@frostming how does this proposal interact with the changes you made here recently?

[frostming]

The change in my PR is: when installing wildcard versions from CLI, Pipfile won't be modified.

With the proposal here, when we do pipenv install mypkg --pre, we are willing to replace the entry, right? The simpliest way is to add an extra arg pre to add_package_to_pipfile.

I am glad to help when it comes true.

[Gr1N]

Hello! Any updates or estimates on this?

[remingtonc]

If there's no progress on this I'd like to make it happen, otherwise I will revert to virtualenv/pip.

@frostming With your proposal, how would that end up looking in the Pipfile? I'm not familiar with how requirements are really parsed, briefly browsed some code. Completely ballparking, please correct me. Would it have to follow syntax akin to:
EDIT: Per initial issue comment:

...
mypkg = { version = "*", pre="true"}
...

[gsemet]

I was on it, but there have been some change in the resolver I need to redo it completly. I would be glad to get some help on it

[jxltom]

I found that pin to specific prerelease version works for even allow_prereleases or --pre is provided