chore: demonstrate py_binary stamping (#532)

Came up from a customer question.

---

Author: alexeagle

MODULE.bazel

@@ -17,6 +17,9 @@ bazel_dep(name = "bazel_skylib", version = "1.4.2")
 bazel_dep(name = "rules_python", version = "0.29.0")
 bazel_dep(name = "platforms", version = "0.0.7")
 
+bazel_lib = use_extension("@aspect_bazel_lib//lib:extensions.bzl", "toolchains")
+bazel_lib.expand_template()
+
 # Custom python version for testing only
 python = use_extension("@rules_python//python/extensions:python.bzl", "python", dev_dependency = True)
 python.toolchain(

WORKSPACE

@@ -42,10 +42,12 @@ py_repositories()
 
 ############################################
 # Aspect bazel-lib
-load("@aspect_bazel_lib//lib:repositories.bzl", "register_coreutils_toolchains")
+load("@aspect_bazel_lib//lib:repositories.bzl", "register_coreutils_toolchains", "register_expand_template_toolchains")
 
 register_coreutils_toolchains()
 
+register_expand_template_toolchains()
+
 load("@musl_toolchains//:repositories.bzl", "load_musl_toolchains")
 
 load_musl_toolchains(extra_target_compatible_with = ["@//tools/linkers:musl"])

examples/py_binary/BUILD.bazel

@@ -1,8 +1,17 @@
+load("@aspect_bazel_lib//lib:expand_template.bzl", "expand_template")
 load("//py:defs.bzl", "py_binary")
 
+expand_template(
+    name = "stamped",
+    stamp_substitutions = {
+        "<BUILD_TIMESTAMP>": "{{BUILD_TIMESTAMP}}",
+    },
+    template = "say.py",
+)
+
 py_binary(
     name = "py_binary",
-    srcs = ["say.py"],
+    srcs = [":stamped"],
     deps = [
         "@pypi_cowsay//:pkg",
     ],

examples/py_binary/say.py

@@ -1,3 +1,3 @@
 import cowsay
 
-cowsay.cow('hello py_binary!')
+cowsay.cow('hello py_binary! (built at <BUILD_TIMESTAMP>)')