Add an example on how to override the URLs

Author: aignas

examples/bzlmod/MODULE.bazel

@@ -42,9 +42,20 @@ python.override(
     available_python_versions = [
         "3.10.9",
         "3.9.19",
+        # Keep the following versions which are present in MINOR_MAPPING, but
+        # are not present above
+        "3.10.14",
     ],
 )
 python.version_override(
+    sha256s = {
+        "aarch64-apple-darwin": "1409acd9a506e2d1d3b65c1488db4e40d8f19d09a7df099667c87a506f71c0ef",
+        "aarch64-unknown-linux-gnu": "8f351a8cc348bb45c0f95b8634c8345ec6e749e483384188ad865b7428342703",
+        "x86_64-apple-darwin": "8146ad4390710ec69b316a5649912df0247d35f4a42e2aa9615bffd87b3e235a",
+        "x86_64-pc-windows-msvc": "a1d9a594cd3103baa24937ad9150c1a389544b4350e859200b3e5c036ac352bd",
+        "x86_64-unknown-linux-gnu": "9b64eca2a94f7aff9409ad70bdaa7fbbf8148692662e764401883957943620dd",
+    },
+    url = "20220227/cpython-{python_version}+20220227-{platform}-{build}.tar.gz",
     version = "3.10.2",
 )
 

python/private/python.bzl

@@ -260,7 +260,8 @@ def _fail_multiple_default_toolchains(first, second):
 def _process_tag_classes(mod):
     arg_structs = []
     seen_versions = {}
-    available_versions = TOOL_VERSIONS
+    available_versions = {}
+    available_versions.update(TOOL_VERSIONS)
     base_url = DEFAULT_RELEASE_BASE_URL
 
     for tag in mod.tags.toolchain:
@@ -270,7 +271,7 @@ def _process_tag_classes(mod):
     if mod.is_root:
         for tag in mod.tags.version_override:
             sha256 = {}
-            for p, sha in tag.sha256.items():
+            for p, sha in tag.sha256s.items():
                 if p not in PLATFORMS:
                     fail("The platform must be one of {allowed} but got '{got}'".format(
                         allowed = sorted(PLATFORMS),
@@ -291,10 +292,10 @@ def _process_tag_classes(mod):
             if tag.available_python_versions:
                 all_known_versions = sorted(available_versions)
                 available_versions = {
-                    v: available_versions.get(v, fail("unknown version {}, known versions: {}".format(
+                    v: available_versions[v] if v in available_versions else fail("unknown version '{}', known versions are: {}".format(
                         v,
                         all_known_versions,
-                    )))
+                    ))
                     for v in tag.available_python_versions
                 }
 
@@ -433,7 +434,7 @@ _version_override = tag_class(
     doc = """Tag class used to override single python version settings.""",
     attrs = {
         "sha256s": attr.string_dict(
-            mandatory = True,
+            mandatory = False,
             doc = "The python platform to sha256 dict. The platform key must be present in the PLATFORMS dict.",
         ),
         "strip_prefix": attr.string(