tweak bazel to use python3

Author: sparkprime

WORKSPACE

@@ -17,6 +17,10 @@ git_repository(
     shallow_since = "1535728917 -0400",
 )
 
-load("//tools/build_defs:python_repo.bzl", "python_interpreter")
+# This allows using py_test and py_library against python3.
+register_toolchains("//platform_defs:default_python3_toolchain")
+
+# This allows building C++ against python3 headers.
+load("//tools/build_defs:python_repo.bzl", "python_headers")
+python_headers(name = "default_python3_headers")
 
-python_interpreter(name = "default_python")

platform_defs/BUILD

@@ -0,0 +1,21 @@
+load("@bazel_tools//tools/python:toolchain.bzl", "py_runtime_pair")
+
+py_runtime(
+    name = "python3",
+    interpreter_path = "/usr/bin/python3",
+    stub_shebang = "#!/usr/bin/env python3",
+    python_version = "PY3",
+)
+
+py_runtime_pair(
+    name = "just_python3",
+    py2_runtime = None,
+    py3_runtime = ":python3",
+)
+
+toolchain(
+    name = "default_python3_toolchain",
+    toolchain = ":just_python3",
+    toolchain_type = "@bazel_tools//tools/python:toolchain_type",
+)
+

python/BUILD

@@ -4,20 +4,22 @@ cc_binary(
     linkshared = 1,
     deps = [
         "//core:libjsonnet",
-        "@default_python//:headers",
+        "@default_python3_headers//:headers",
     ],
 )
 
 py_library(
     name = "_jsonnet",
     data = [":_jsonnet.so"],
     imports = ["."],
+    srcs_version = "PY3",
     visibility = ["//visibility:public"],
 )
 
 py_test(
     name = "_jsonnet_test",
     srcs = ["_jsonnet_test.py"],
     data = ["test.jsonnet"],
+    python_version = "PY3",
     deps = [":_jsonnet"],
 )

tools/build_defs/python_repo.bzl

@@ -1,12 +1,6 @@
 build_file_contents = """\
 package(default_visibility = ["//visibility:public"])
 
-sh_binary(
-    name = "python",
-    srcs = ["bin/python"],
-    data = [":include"],
-)
-
 filegroup(
     name = "include",
     srcs = glob(["include/**"]),
@@ -19,7 +13,7 @@ cc_library(
 )
 """
 
-def _python_interpreter(repository_ctx):
+def _impl(repository_ctx):
   rctx = repository_ctx
   if "/" in rctx.attr.path or "\\" in rctx.attr.path:
     # Canonicalize the path
@@ -41,12 +35,12 @@ def _python_interpreter(repository_ctx):
   rctx.file("BUILD", build_file_contents)
 
 
-python_interpreter = repository_rule(
-    implementation = _python_interpreter,
+python_headers = repository_rule(
+    implementation = _impl,
     local = True,
     attrs = {
         "path": attr.string(
-            default = "python",
+            default = "python3",
         ),
     },
 )