basic custom python 3 toolchain setup with bazel

.bazelversion

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+3.3.1

WORKSPACE

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+workspace(name = "custom_python_toolchain")
+
+load("//tools/python:toolchain.bzl", "register_python_toolchain")
+register_python_toolchain()
+
+load("//tools/python:python.bzl", "rules_python")
+rules_python()

tests/BUILD.bazel

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+load("//tools/python:py3_macros.bzl", "py3_test")
+
+py3_test(
+    name = "toolchain_test",
+    srcs = [
+        "toolchain_test.py",
+    ],
+)

tests/toolchain_test.py

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+import unittest
+import sys
+
+
+class PythonToolchainRuntimeTest(unittest.TestCase):
+
+    def test_toolchain_runtime_interpreter(self):
+        interpreter_path = python3_path()
+        print(f"Interpreter path: {interpreter_path}")
+        self.assertTrue(interpreter_path.__contains__("execroot/custom_python_toolchain"))
+
+
+def python3_path():
+    return sys.executable
+
+
+if __name__ == '__main__':
+    unittest.main()

tools/python/BUILD.bazel

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+load("@bazel_tools//tools/python:toolchain.bzl", "py_runtime_pair")
+
+py_runtime(
+    name = "python3_runtime",
+    files = ["@python_interpreter//:files"],
+    interpreter = "@python_interpreter//:python_bin",
+    python_version = "PY3",
+    visibility = ["//visibility:public"],
+)
+
+py_runtime_pair(
+    name = "sourced_py_runtime_pair",
+    py2_runtime = None,
+    py3_runtime = ":python3_runtime",
+)
+
+toolchain(
+    name = "py_toolchain",
+    toolchain = ":sourced_py_runtime_pair",
+    toolchain_type = "@bazel_tools//tools/python:toolchain_type",
+)

tools/python/py3_macros.bzl

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+load("@rules_python//python:python.bzl", "py_binary", "py_library", "py_test")
+
+def py3_library(*args, **kwargs):
+    py_library(
+        srcs_version = "PY3",
+        *args,
+        **kwargs
+    )
+
+def py3_binary(name, main = None, *args, **kwargs):
+    if main == None:
+        main = "%s.py" % (name)
+
+    py_binary(
+        name = name,
+        main = main,
+        legacy_create_init = False,
+        python_version = "PY3",
+        *args,
+        **kwargs
+    )
+
+def py3_test(*args, **kwargs):
+    py_test(
+        legacy_create_init = False,
+        python_version = "PY3",
+        srcs_version = "PY3",
+        *args,
+        **kwargs
+    )

tools/python/python.bzl

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+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+rules_python_version = "dd7f9c5f01bafbfea08c44092b6b0c8fc8fcb77f"
+rules_python_sha256 = "0aa9ec790a58053e3ab5af397879b267a625955f8297c239b2d8559c6773397b"
+
+
+def rules_python():
+    http_archive(
+      name = "rules_python",
+      sha256 = rules_python_sha256,
+      strip_prefix= "rules_python-%s" % rules_python_version,
+      urls = ["https://github.com/bazelbuild/rules_python/archive/%s.tar.gz" % rules_python_version],
+    )

tools/python/toolchain.bzl

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+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+_interpreter_version = "3.8.5"
+_interpreter_package_sha256 = "e3003ed57db17e617acb382b0cade29a248c6026b1bd8aad1f976e9af66a83b0"
+
+
+def register_python_toolchain():
+    # NOTE: macOS's the default openssl binaries are not sufficient to build the SSL module. It is possible to change
+    # the patch command to configure the build to use a custom SSL library by specifying '--with-openssl=<SSL lib path>'
+    # to the configure script:
+    #
+    #       "./configure --prefix=$(pwd)/python_home --with-openssl=/path/to/openssl/1.1",
+    #
+    #
+    # Here is the message yield by the build on macOS:
+    #
+    #   Could not build the ssl module!
+    #   Python requires an OpenSSL 1.0.2 or 1.1 compatible libssl with X509_VERIFY_PARAM_set1_host().
+    #   LibreSSL 2.6.4 and earlier do not provide the necessary APIs, https://github.com/libressl-portable/portable/issues/381
+    #
+    http_archive(
+        name = "python_interpreter",
+        urls = [
+            "https://www.python.org/ftp/python/%s/Python-%s.tar.xz"%(_interpreter_version, _interpreter_version)
+        ],
+        sha256 = _interpreter_package_sha256,
+        strip_prefix = "Python-%s"%_interpreter_version,
+        patch_cmds = [
+            "mkdir $(pwd)/python_home",
+            "./configure --prefix=$(pwd)/python_home",
+            "make",
+            "make install",
+            "ln -s python_home/bin/python3 python_bin",
+        ],
+        build_file_content = """
+exports_files(["python_bin"])
+
+filegroup(
+name = "files",
+srcs = glob(["python_home/**"], exclude = ["**/* *"]),
+visibility = ["//visibility:public"],
+)
+""",
+    )
+
+    native.register_toolchains("//tools/python:py_toolchain")