feat: renders record their answers, so copier update can reach them

.github/workflows/ci.yml

@@ -67,10 +67,49 @@ jobs:
           uv run ruff check tests/unit/test_probe.py
           rm demo_lib/probe.py tests/unit/test_probe.py
 
+  # A library takes template changes with `copier update`, so a change has to
+  # arrive that way too: a render of master, updated to this commit, has to
+  # come out exactly as a fresh render of this commit.
+  update:
+    name: copier update brings the change into a render of master
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          fetch-depth: 0
+      - uses: astral-sh/setup-uv@v7
+
+      - name: Update a render of master to this commit
+        run: |
+          git fetch --no-tags origin master:refs/heads/ci-base
+          git branch -f ci-head HEAD
+          if ! git cat-file -e 'ci-base:template/{{_copier_conf.answers_file}}.jinja' 2>/dev/null; then
+            echo "A render of master has no answers file to update from."
+            exit 0
+          fi
+          render() {
+            uvx --from copier==9.19.1 --with jinja2-time==0.2.0 copier copy --quiet --defaults --trust --vcs-ref "$1" \
+              --data project_name=demo-lib \
+              --data project_slug=demo-lib \
+              --data package_name=demo_lib \
+              --data project_description="Demo library rendered from the template" \
+              --data author_name="Bedrock Python" \
+              --data author_email="maintainers@example.com" \
+              "$GITHUB_WORKSPACE" "$2"
+          }
+          render ci-base "$RUNNER_TEMP/updated"
+          render ci-head "$RUNNER_TEMP/fresh"
+          cd "$RUNNER_TEMP/updated"
+          git init -q -b master
+          git add -A
+          git -c user.name=ci -c user.email=ci@example.com commit -q -m "render of master"
+          uvx --from copier==9.19.1 --with jinja2-time==0.2.0 copier update --quiet --defaults --trust --vcs-ref ci-head
+          diff -r --exclude=.git "$RUNNER_TEMP/fresh" "$RUNNER_TEMP/updated"
+
   all-checks-passed:
     name: All checks passed
     if: always()
-    needs: [render]
+    needs: [render, update]
     runs-on: ubuntu-latest
     steps:
       - name: Every job above succeeded

README.md

@@ -59,6 +59,24 @@ Then:
 4. Set up PyPI Trusted Publisher at pypi.org/manage/account/publishing/
 5. Add `CODECOV_TOKEN` secret in repo Settings → Secrets → Actions
 
+## Pulling template changes into a library
+
+A render records its answers and the template commit it came from in
+`.copier-answers.yml`. Commit that file and leave it to Copier. To bring a library up to
+the template's `master`, from the library's clean working tree:
+
+```bash
+uvx --with jinja2-time copier update --trust --defaults
+```
+
+Copier applies what changed in the template since the recorded commit as a three-way
+merge with the library's own edits, leaves conflicts as inline markers, and moves
+`_commit` forward. A file the library deleted, such as the checklist, stays deleted.
+
+A library rendered before the answers file existed needs one written by hand first: the
+answers it was rendered with, `_src_path: gh:bedrock-python/python-library-template`,
+and as `_commit` the template commit it was rendered from, or the closest one.
+
 ## For AI Agents
 
 See [`.claude/LIBRARY_CREATION.md`](.claude/LIBRARY_CREATION.md) for important rules:

copier.yml

@@ -45,13 +45,16 @@ _jinja_extensions:
   - jinja2_time.TimeExtension
 
 _tasks:
-  - >-
-    echo "Template generated. Next steps:
-    1. uv sync --group dev
-    2. uv run pre-commit install --hook-type commit-msg
-    3. Create the GitHub repo and push (branch: master)
-    4. After the first CI pass, from the template repo:
-    python scripts/setup_repo.py {{ github_org }}/{{ project_slug }}
-    5. Configure the PyPI Trusted Publisher at pypi.org
-    6. Add the CODECOV_TOKEN secret.
-    See NEW_LIBRARY_CHECKLIST.md for full details."
+  # On a copy only: `copier update` runs tasks too, and a library taking a
+  # template update finished these steps long ago.
+  - command: >-
+      echo "Template generated. Next steps:
+      1. uv sync --group dev
+      2. uv run pre-commit install --hook-type commit-msg
+      3. Create the GitHub repo and push (branch: master)
+      4. After the first CI pass, from the template repo:
+      python scripts/setup_repo.py {{ github_org }}/{{ project_slug }}
+      5. Configure the PyPI Trusted Publisher at pypi.org
+      6. Add the CODECOV_TOKEN secret.
+      See NEW_LIBRARY_CHECKLIST.md for full details."
+    when: "{{ _copier_operation == 'copy' }}"

template/{{_copier_conf.answers_file}}.jinja

@@ -0,0 +1,2 @@
+# Changes here will be overwritten by Copier; NEVER EDIT MANUALLY
+{{ _copier_answers|to_nice_yaml -}}