default py_runtime.implementation_name to cpython

Author: rickeylev

CHANGELOG.md

@@ -25,7 +25,8 @@ A brief description of the categories of changes:
 [x.x.x]: https://github.com/bazelbuild/rules_python/releases/tag/x.x.x
 
 ### Changed
-* Nothing yet
+* (toolchains) `py_runtime.implementation_name` now defaults to `cpython`
+  (previously it defaulted to None).
 
 ### Fixed
 * (bzlmod) correctly wire the {attr}`pip.parse.extra_pip_args` all the

python/private/common/py_runtime_rule.bzl

@@ -226,6 +226,7 @@ runtime. For a platform runtime this attribute must not be set.
         ),
         "implementation_name": attr.string(
             doc = "The Python implementation name (`sys.implementation.name`)",
+            default = "cpython",
         ),
         "interpreter": attr.label(
             # We set `allow_files = True` to allow specifying executable