[Distutils] PEP345 - Distutils2 - Cannot pin to exact version

[Daniel Holth]

On Tue, Sep 11, 2012 at 3:11 PM, Erik Bray <erik.m.bray at gmail.com> wrote:
> On Tue, Sep 11, 2012 at 2:35 PM, Donald Stufft <donald.stufft at gmail.com> wrote:
>> I was digging through PEP386 & PEP345 today, and I noticed something odd
>> about the wording of PEP345.
>>
>> It states:
>>
>>     When a version is provided, it always includes all versions that starts
>> with the same value. For
>>     example the "2.5" version of Python will include versions like "2.5.2"
>> or "2.5.3". Pre and post
>>     releases in that case are excluded. So in our example, versions like
>> "2.5a1" are not included
>>     when "2.5" is used. If the first version of the range is required, it
>> has to be explicitly given. In
>>     our example, it will be "2.5.0".
>>
>> It also states:
>>
>>     In that case, "2.5.0" will have to be explicitly used to avoid any
>> confusion between the "2.5"
>>     notation that represents the full range. It is a recommended practice to
>> use schemes of the
>>     same length for a series to completely avoid this problem.
>>
>> This effectively translates to an inability to pin to an exact version. Even
>> in the case of specifying
>> == it checks that the version "starts with" the value you selected. So if
>> you pin to "2.5", and the
>> author then releases "2.5.1", that will count as ==2.5. If you try to then
>> pin to "2.5.0", and the
>> author releases "2.5.0.1", then that will count as ==2.5.0.
>>
>> Essentially this translates to:
>>
>>     ==2.5       -> >=2.5<2.6
>>     ==2.5.0    -> >=2.5.0<2.5.1
>>     ==2.5.0.0 -> >=2.5.0.0<2.5.0.1
>>
>> Which means that version specifiers are _always_ ranges and are never exact
>> versions. The PEP
>> as written relies on authors to decide beforehand how many digits they are
>> going to use in their
>> versions, and for them to never increase or decrease that number.
>>
>> I also checked to see if Distutils2/packaging implemented VersionPredicates
>> that way or if they
>> allowed specifying an exact version. It turned out that it implements the
>> PEP as written:
>>
>>>>> from distutils2 import version
>>>>> predicate = version.VersionPredicate("foo (==2.5)")
>>>>> print predicate
>> foo (==2.5)
>>>>> predicate.match("2.5")
>> True
>>>>> predicate.match("2.5.0")
>> True
>>>>> predicate.match("2.5.0.0")
>> True
>>>>> predicate.mach("2.5.0.5")
>> True
>
> That's kind of annoying.  Does anyone know if this is by design?
>
> FWIW there is a workaround. For example if you want to pin to exactly 2.5.1:
>
>>>> predicate = version.VersionPredicate("foo (==2.5.1,<2.5.1.1)")
>>>> predicate.match('2.5.1')
> True
>>>> predicate.match('2.5.2')
> False
>>>> predicate.match('2.5.1.0')
> True
>>>> predicate.match('2.5.1.1')

But you could still release 2.5.1.0.0? I suppose we limit the number
of version parts these days.

Why don't we update the spec so that (2.0) means (2.0) the range, and
(==2.0) means 2.0 (exactly).

Daniel