What's the difference between a shell environment and a profile?

[Konrad Hinsen]

Hi Guix experts,

While doing some software archeology, I ran into a behavior that I don't
quit understand.

First, I tested some Python 2 scripts in a shell environment:

    guix time-machine -C ./channels.scm \
        -- shell --container \
           python2 python2-mmtk python2-matplotlib \
           --with-input=python2-numpy=python2-numpy@1.8.2 \
           -- python

Works fine. Next, I wanted to create a Guix profile with the same
software, mainly to keep it in the store for long-term future use,
surviving garbage collection:

    guix time-machine -C ./channels.scm \
        -- package -p ./python2-profile \
           -i python2 python2-mmtk python2-matplotlib \
           --with-input=python2-numpy=python2-numpy@1.8.2

Guix wasn't happy at all with this:

  guix package: warning: Consider running 'guix pull' followed by
  'guix package -u' to get up-to-date packages and security updates.

  The following packages will be installed:
     python2            2.7.18
     python2-matplotlib 2.2.5
     python2-mmtk       2.7.13

  guix package: error: profile contains conflicting entries for glib
  guix package: error:   first entry: glib@2.70.2 
/gnu/store/64bdjb3nwdkadmy5z2wph9cgqr0bwijm-glib-2.70.2
  guix package: error:    ... propagated from cairo@1.16.0
  guix package: error:    ... propagated from python2-pycairo@1.18.2
  guix package: error:    ... propagated from python2-matplotlib@2.2.5
  guix package: error:   second entry: glib@2.73.3 
/gnu/store/kf488k7v0lc48ylbs4xxpam0dbl3r4jl-glib-2.73.3
  guix package: error:    ... propagated from gobject-introspection@1.73.1
  guix package: error:    ... propagated from python2-matplotlib@2.2.5
  hint: You cannot have two different versions or variants of 
`python2-matplotlib' in
the same profile.

In the end I did:

    guix time-machine -C ./channels.scm \
        -- shell --container -r ./python2-profile \
           python2 python2-mmtk python2-matplotlib \
           --with-input=python2-numpy=python2-numpy@1.8.2 \
           -- python

which should have the effect I am looking for, but I don't understand
how this differs from the "guix package" attempt that failed. The
difference seems to be related to the –with-input transform, because
I don't see what else could cause the "two different versions of
python2-matplotlib" to be around.

Any ideas?

Cheers,
  Konrad