[Distutils] IBM Developerworks setuptools article - example doesnt work ?

[Bob Ippolito]

On 10/30/06, Michael Bayer <mike_mp at zzzcomputing.com> wrote:
> I just read an IBM Developerworks article on setuptools today, its
> linked at:
>
> http://www-128.ibm.com/developerworks/linux/library/l-cppeak3.html?
> ca=dgr-lnxw07PythonEggWithSetuptools
>
> it refers to the ability to use pkg_resources.require() in a Python
> script to specify any particular version of a package that happens to
> exist in site-packages, overriding the currently active
> package...something i used to think setuptools could do, but then
> figured it couldnt (even though it seems like obviously useful
> functionality), and it seems that it still cannot.  from the
> article's example:
>
> # install Gnosis_Utils (latest version is 1.2.1)
> easy_install -f http://gnosis.cx/download/Gnosis_Utils.More/
> Gnosis_Utils
>
> # install version 1.2.0 of Gnosis_Utils
> easy_install -f http://gnosis.cx/download/Gnosis_Utils.More/
> "Gnosis_Utils==1.2.0"
>
> # re-activate version 1.2.1
> easy_install "Gnosis_Utils==1.2.1"
>
> # run a script that requires version 1.2.0
> from pkg_resources import require
> require("Gnosis_Utils==1.2.0")
>
> Traceback (most recent call last):
>    File "<stdin>", line 1, in ?
>    File "/Library/Frameworks/Python.framework/Versions/2.4/lib/
> python2.4/site-packages/setuptools-0.6c3-py2.4.egg/pkg_resources.py",
> line 585, in require
>      needed = self.resolve(parse_requirements(requirements))
>    File "/Library/Frameworks/Python.framework/Versions/2.4/lib/
> python2.4/site-packages/setuptools-0.6c3-py2.4.egg/pkg_resources.py",
> line 487, in resolve
>      raise VersionConflict(dist,req) # XXX put more info here
> pkg_resources.VersionConflict: (Gnosis-Utils 1.2.1 (/Library/
> Frameworks/Python.framework/Versions/2.4/lib/python2.4/site-packages/
> Gnosis_Utils-1.2.1-py2.4.egg), Requirement.parse('Gnosis-Utils==1.2.0'))
>
>
> bug ?  or feature ?

Feature. It can do multi-version installs, but only if explicitly specified.

The problem is that 1.2.1 is already on sys.path, so setuptools gives
up because it doesn't know if something is using Gnosis-Utils or not.
If the directory *containing* the egg was on sys.path, then it would
pick the exact version... but then you must *always* require it before
it is imported.

-bob