Support testing against Python pre-releases on Windows

[jaraco]

In msg369233, @zooba suggests that it should be possible to test against Python pre-releases. The suggestion there was to run something like:

nuget install python -Prerelease -OutputDirectory . -ExcludeVersion
.\python\tools\python.exe ...

However, that recommendation doesn't really fit into the azure pipelines config the skeleton projects use. This uses UsePythonVersion. Any idea how to run pre-releases with that plugin? Does it automatically support pre-released versions (such as 3.9 now)?

[jaraco]

It seems that tool, although it mentions prereleases in its code, doesn't support prereleases.

##[error]Version spec 3.9 for architecture x64 did not match any version in Agent.ToolsDirectory.
Versions in C:/hostedtoolcache/windows:
2.7.18 (x86)
3.5.4 (x86)
3.6.8 (x86)
3.7.7 (x86)
3.8.2 (x86)
2.7.18 (x64)
3.5.4 (x64)
3.6.8 (x64)
3.7.7 (x64)
3.8.2 (x64)

I'd like some help devising a proper fix to support pre-release Pythons.

[zooba]

The built-in plugin only exposes versions that have been added to the tool cache on the build machine. For the hosted VMs, you have to request it and wait (which I've already done).

The code you'll actually need for now is going to be more like this (untested, but a good starting point for someone who's got time to work on this):

    - powershell: |
        nuget install python -Prerelease -OutputDirectory "$(Build.BinariesDirectory)" -ExcludeVersion -NonInteractive
        Write-Host "##vso[task.prependpath]$(Build.BinariesDirectory)\python\tools"
        Write-Host "##vso[task.prependpath]$(Build.BinariesDirectory)\python\tools\Scripts"
      condition: and(succeeded(), and(eq($(python.version), '3.9')), eq($(pool_vm_image), 'vs2017-win2016'))
    - task: UsePythonVersion@0
      inputs:
        versionSpec: '$(python.version)'
        architecture: 'x64'
      condition: and(succeeded(), ne($(python.version), '3.9'))

[jaraco]

In jaraco/path@ab8b1d7, I've rebalanced the parentheses for the first condition using this interpretation:

and(
	succeeded(),
	and(
		eq(
			$(python.version),
			'3.9'
		),
		eq(
			$(pool_vm_image),
			'vs2017-win2016'
		)
	)
)

And now I'm getting this error:

##[error]Unrecognized value: '$'. Located at position 25 within expression: and(succeeded(), and(eq($(python.version), '3.9'), eq($(pool_vm_image), 'vs2017-win2016'))). For more help, refer to https://go.microsoft.com/fwlink/?linkid=842996

Position 25 is the first $ at $(python.version).

Based on the syntax in the Publish stage, I suspect that should be variables['python.version']. I'll try that next.

[jaraco]

In jaraco/path@fd5f8fe, I'm getting closer. The conditions seem to be parsing correctly, but now the error is:

nuget : The term 'nuget' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.

There I'm stuck. I searched for "nuget Azure pipelines", but all the resources seem to be about publishing to nuget, not consuming. I looked at the generic docs for nuget, but they suggest downloading the exe. My guess is that I shouldn't be manually downloading the installer. I'll await more guidance.

[zooba]

Ah yeah, good catch on the variable syntax.

I thought Nuget was already public? But perhaps not. It should be available in the tools cache though, so just adding this at the start (with the same condition to limit it to Windows) will do the PATH update:

- task: NuGetToolInstaller@1
  displayName: 'if you want one'

[jaraco]

Success! Thanks for the guidance. I should be able to use this to catch behaviors early for Python 3.9 across many of my projects.