Python support requires pip or pypi

[pcj]

In order to support python fully in rules_protobuf, support for pulling dependencies from pip or pypi is required. This is non-trivial for grpc as there is c-compilation involved with package installation. Requires bazelbuild/bazel#699 or similar.

[pcj]

Watching https://github.com/gengo/rules_pypi.

[pcj]

Now watching bazel-contrib/rules_python#1

[mattmoor]

@pcj Can you try this out and make sure it works for your use case?

As an avid Bazel-er, I'd love your feedback on the general workflow (e.g. the pattern for using requirements.txt as-is to bridge the world of Python idiomatic with Bazel). I've been debating something similar for Node.js and package-lock.json.

[pcj]

@mattmoor Sure happy to.

One nit I saw was in https://github.com/mattmoor/rules_python/tree/07fba0f91bb5898d19daeaabf635d08059f7eacd where load("//python:pip.bzl", "pip_repositories") should read load("//@io_bazel_rules_python//python:pip.bzl", "pip_repositories")

[pcj]

As far as general workflow, I like it. I implemented a similar 2-stage loading pattern in https://github.com/pubref/rules_maven that I think will become common in bazel eventually.

[pcj]

@mattmoor Worked great! I've pushed a new PR to this repo that uses your new rules and adds grpcio support here for python.

I'll let this PR simmer for a bit until you and Damien work through your PR and update/merge it once you push those rules to master.

I took a look as well but didn't see anything to comment on, so it's good IMHO.