feat: Initial draft of GAPIC Bazel Extensions gapic-generator-python

[vam-google]

This is a "minimal viable product" version of the rule, which simply call the python microgenerator to generate a client. No post and/or preprocessing is implemented, but the actual GAPIC code is produced successfully.

Successfully tested on googleapis/google/cloud/lanugage/v1 API.

To use it from googleapis, add the following to the # Python in WORKSPACE file in googleapis:

load("@rules_python//python:repositories.bzl", "py_repositories")
py_repositories()

load("@rules_python//python:pip.bzl", "pip_repositories")
pip_repositories()

# Change upstream repository once PR is merged
http_archive(
    name = "gapic_generator_python",
    urls = ["https://github.com/vam-google/gapic-generator-python/archive/77637d9bfdbfcb67b02b3d6bbbc6938de44214ce.zip"],
    strip_prefix = "gapic-generator-python-77637d9bfdbfcb67b02b3d6bbbc6938de44214ce",
)

load("@gapic_generator_python//:repositories.bzl", "gapic_generator_python")

gapic_generator_python()

load("@gapic_generator_python_pip_deps//:requirements.bzl", "pip_install")
pip_install()

and the following at the top of the file (needs to go to the top, because rules_python of older version is a transitive dependency of grpc rules, we need a newer version to be used in workspace, so specify it first to override the older one).

http_archive(
    name = "rules_python",
    url = "https://github.com/bazelbuild/rules_python/archive/748aa53d7701e71101dfd15d800e100f6ff8e5d1.zip",
    strip_prefix = "rules_python-748aa53d7701e71101dfd15d800e100f6ff8e5d1"
)

The in google/cloud/langugae/v1/BUILD.bazel file in Python section add

load("@gapic_generator_python//rules_python_gapic:py_gapic.bzl", py_gapic_library2 = "py_gapic_library")

py_gapic_library2(
    name = "language_py_gapic2",
    srcs = [":language_proto"],
)

and build it with:

bazel build //google/cloud/language/v1:language_py_gapic2

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[software-dov]

Looks good so far, just a few questions.

[software-dov]

Is there any way we can be more specific about the python version requirement? Some of the code in the generator requires features from >=3.6

[vam-google]

There are ways to be more specific, but they have their own issues (i.e. make the build less hermetic and less portable). Practically speaking, it should be picking the latest installed version of python on the host machine, and if there is 3.6+ bazel will pick it up.

[software-dov]

Okay. Good to know.

[software-dov]

Can we pass custom options through **kwargs? Is it straightforward?

[vam-google]

yes we can, we can also always add those custom options explicitly as args in this rule. It is straightforward to do (I did not add any because of my lack of knowledge and context here).

[software-dov]

Excellent. We will try using kwargs first and see if explicit args are necessary.

[vam-google]

@software-dov I cleaned up this PR, and added instructions how it can be used from googleapis to test stuff, PTAL.

[software-dov]

LGTM. One question: I don't see a hash anywhere that corresponds to gapic-generator-python; other things yes, but not that one. Does the rule automatically use the remote repository master? Do I need to update anything if I cut a new release?

[vam-google]

@software-dov Maybe there is some misunderstanding how this things (bazel rules and bazel workspaces) work together.

There is a workspace which defines a rule (this one, gapic-generator-python defines the py_gapic_library rule), and a workspace which uses the rule (it often can be same repo, but for our case it is the googleapis repository). To wire these two things together, googleapis in its WORKSPACE file defines the actual version of gapic-generator-python repository (i.e. the one, which defines the rule) to pull. In the instructions provided in this PR description it corresponds to the following lines:

http_archive(
    name = "gapic_generator_python",
    urls = ["https://github.com/vam-google/gapic-generator-python/archive/77637d9bfdbfcb67b02b3d6bbbc6938de44214ce.zip"],
    strip_prefix = "gapic-generator-python-77637d9bfdbfcb67b02b3d6bbbc6938de44214ce",
)

Where 77637d9bfdbfcb67b02b3d6bbbc6938de44214ce is literally the github commit ID. It does not have to be a github commit id, it can be a versioned release tag, for examle (if there is any). That is it, you do not need to cut any releases of anything (but still can, if you want). When you update something in gapic-generator-python and want to use it in googleapis, you just need to update commit id, or, even simly use local_repository rule to pull it (so your local build will simply pull stuff from your harddrive).

[vam-google]

@software-dov Pleas push this PR if it looks good to you (i don't have rights).

[software-dov]

Okay, so if I do cut new releases (or make new commits on top of master) I need to change the hash in googleapis' workspace file. Got it.