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

[Donald Stufft]

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

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