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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