[TIP] Why does tox use "pip install --pre" by default?

[Ned Batchelder]

On 9/27/14 2:24 PM, holger krekel wrote:
> On Sat, Sep 27, 2014 at 13:46 -0400, Ned Batchelder wrote:
>> I tried to release an alpha version of coverage.py 4.0 today, and
>> immediately got complaints about Travis builds failing.
>> It turns out that tox's default install_command is "pip install --pre
>> {opts} {packages}".
> Did you you release the alpha to pypi.python.org and then
> other packages using coverage via tox started failing?
Yes.  coveralls.io (unwisely) uses internals of coverage.py which have 
changed in the major refactor that was part of 4.0.  So coveralls breaks 
when it tries to report statistics.  Anyone using tox and coveralls.io 
would have seen a failure.
>
> When i release beta/devs of my packages i typically release
> them to a public index other than pypi.python.org
> (e.g. https://devpi.net/hpk/dev/ ).
I don't have another index I can use, unless they are out there and I 
don't know they exist.
>
>> Why the --pre ?  Do most people want to use beta
>> versions of dependencies in their tests?
> a) I (and i think many others) often use tox to test against
>     other packages development packages or release candidates.
The real question is, "what does a naive user of tox expect?"  You want 
to test against development packages, so you will make sure that 
happens.  But if someone hasn't put in the effort to learn and/or change 
the defaults, what do they expect to happen?  I would have thought that 
tox by default should use pip's defaults.  I see no reason why running 
tests in a number of scenarios (which is tox's job) should mean that I 
want to install software in an unusual way. Certainly it's good to be 
able to test against development packages, and overriding the tox 
defaults would be a way to do that.

But by default, I think tox should use pip's defaults.
>
> b) when we introduced "install_command" the newest version of pip
>     was just changing defaults to not install dev/rc/beta packages
>     so the "install whatever is there" was still what most people
>     expected.
>
> In any case, I wouldn't like to change defaults at this stage now
> unless there is a strong reason for it.  It would break my own setups
> and probably many others.
I certainly understand that mindset.  But I don't agree that it would 
"break" a setup.  Your code would continue to test against all the same 
packages, but it would use the latest released package rather than the 
latest pre-release package.  If your code will only run against the 
latest pre-release package, instead of the latest released package, then 
you need a version specifier in your requirements.

People who preferred to always use pre-release packages would be able to 
change their tox.ini to continue doing so.

Changing defaults to make them safer is a time-honored tradition. Pip 
did it, as you point out, and we were all fine.

--Ned.
>
> cheers,
> holger
>
>> Of course, there's a slew of things that had to go wrong for the
>> failures, notably that coveralls.io is using internal things from
>> coverage that are no longer there, which is what actually caused the
>> failures.  Also, "pip install --pre" will install even versions that
>> are hidden on PyPI.  :(
>>
>> --Ned.
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