Add flavor_setting

BUILD

@@ -2,6 +2,8 @@ load("@rules_python//python:defs.bzl", "py_binary")
 load("@rules_python//python:defs.bzl", "py_runtime_pair")
 load("@pip_requirements//:requirements.bzl", "requirement")
 
+load("//:build_settings.bzl", "flavor")
+
 py_runtime(
     name = "python37",
     interpreter_path = "/usr/local/bin/python3.7",
@@ -25,14 +27,34 @@ toolchain(
     toolchain = ":py_runtime_pair",
     toolchain_type = "@rules_python//python:toolchain_type",
 )
+
+flavor(
+    name = "favorite_flavor",
+    build_setting_default = "ORANGE"
+)
+
+config_setting(
+    name = "apple_flavor_setting",
+    flag_values = {
+        ":favorite_flavor": "APPLE"
+    },
+)
+
+config_setting(
+    name = "banana_flavor_setting",
+    flag_values = {
+        ":favorite_flavor": "BANANA"
+    },
 )
 
 py_binary(
     name = "main",
     srcs = ["main.py"],
-    args = [
+    args = select({
-        "--cool",
+        ":apple_flavor_setting": ["--red", "--green"],
-    ],
+        ":banana_flavor_setting": ["--brown", "--curved=true"],
+        "//conditions:default": ["--orange"],
+    }),
     srcs_version = "PY3",
     deps = [
         requirement("pyyaml"),

README.md

@@ -0,0 +1,9 @@
+# Bazel playground
+
+## Play with ...
+
+- `bazel run main --//:favorite_flavor="BANANA"`
+- `bazel run main --//:favorite_flavor="APPLE"`
+- `bazel run main --//:favorite_flavor="ORANGE"`
+- `bazel run main`
+

build_settings.bzl

@@ -0,0 +1,18 @@
+# https://docs.bazel.build/versions/2.1.0/skylark/config.html
+
+FlavorProvider = provider(fields = ['type'])
+
+flavors = ["APPLE", "BANANA", "ORANGE"]
+
+def _impl(ctx):
+    raw_flavor = ctx.build_setting_value
+    if raw_flavor not in flavors:
+        fail(str(ctx.label) + " build setting allowed to take values {"
+             + ", ".join(flavors) + "} but was set to unallowed value "
+             + raw_flavor)
+    return FlavorProvider(type = raw_flavor)
+
+flavor = rule(
+    implementation = _impl,
+    build_setting = config.string(flag = True)
+)