Why not with python.pkgs [ numpy scipy ... ]?

I did try that initially, but protobuf seemed to clash with the above import (didn't install).
Perhaps I messed up something else, but this was my workaround

Please cherry-pick matthiasbeyer@879db97

With this patch, caffe still builds for me.

The build works regardless of the clash, simply start up python and import caffe.

Python 3.7.3 (default, Mar 25 2019, 20:59:09)
[GCC 7.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import caffe
Failed to include caffe_pb2, things might go wrong!
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/nix/store/30n7msvdrqhj4ab9151rp402j50vdsck-caffe-1.0/lib/python3.7/site-packages/caffe/__init__.py", line 4, in <module>
    from .proto.caffe_pb2 import TRAIN, TEST
  File "/nix/store/30n7msvdrqhj4ab9151rp402j50vdsck-caffe-1.0/lib/python3.7/site-packages/caffe/proto/caffe_pb2.py", line 7, in <module>
    from google.protobuf.internal import enum_type_wrapper
ModuleNotFoundError: No module named 'google'

How do you even import this, the package seems to provide only binaries, no libraries... or am I completely missing something here?

You don't need to specify python3 as an extra package, python3Packages.xxx internally requires it.
I'm on nixos 19.03. Try adding the pure arg --pure to nix-shell. Your patch is not working for me.

$ nix-shell -p python3Packages.caffe --pure --run "python"

And then importing caffe works for me, I'm on centos7 on my testing machine. Fun things... maybe someone can explain why this works for me...

If you're running centos 7, do you have python bindings protobuf elsewhere in the env? rpm -q protobuf-python perhaps? Maybe try importing google and print(google.__file__). Aside from that, I have no idea

Nope, none of that. Also, --pure would prevent leaking stuff into the environment, right?

If this flag is specified, the environment is almost entirely cleared before the interactive shell is started, so you get an environment that more closely corresponds to the “real” Nix build. A few variables, in particular HOME, USER and DISPLAY, are retained. Note that ~/.bashrc and (depending on your Bash installation) /etc/bashrc are still sourced, so any variables set there will affect the interactive shell.

AKA, if PYTHONPATH is leaking in somehow, that might explain why.
I'm off to sleep now friend. G'night