Experiment with determining GitHub branch name

example_docs/docs/conf.py

@@ -11,6 +11,7 @@
 # that they have been altered from the originals.
 
 import os
+import re
 import sys
 
 # This allows autodoc to find the `api_example` folder.
@@ -100,3 +101,31 @@
     # Default image for thumbnails.
     "**": "_static/images/logo.png",
 }
+
+
+def determine_github_branch() -> str:
+    """Determine the GitHub branch name to use for source code links.
+
+    We need to decide whether to use `stable/<version>` vs. `main` for dev builds. Refer to https://docs.github.com/en/actions/learn-github-actions/variables for how we determine this with GitHub Actions.
+    """
+    # If not `GITHUB_REF_NAME` is not set, default to `main`. This
+    # is relevant for local builds.
+    if "GITHUB_REF_NAME" not in os.environ:
+        return "main"
+
+    # PR workflows set the branch they're merging into.
+    if base_ref := os.environ.get("GITHUB_BASE_REF"):
+        return base_ref
+
+    ref_name = os.environ["GITHUB_REF_NAME"]
+    if os.environ["GITHUB_REF_TYPE"] == "branch":
+        return ref_name
+
+    # Else, the ref_name is a tag like `1.0.0` or `1.0.0rc1`. We need
+    # to transform this to a Git branch like `stable/1.0`.
+    version_without_patch = re.match(r"(\d+\.\d+)", ref_name).group()
+    return f"stable/{version_without_patch}"
+
+
+GITHUB_BRANCH = determine_github_branch()
+raise ValueError(GITHUB_BRANCH)