feat: allow templating _answers_file setting (#1027)

Co-authored-by: Ethan Barker <ethanbarker0@gmail.com>

copier/main.py

@@ -410,7 +410,9 @@ def answers_relpath(self) -> Path:
         2. Template default.
         3. Copier default.
         """
-        return self.answers_file or self.template.answers_relpath
+        path = self.answers_file or self.template.answers_relpath
+        template = self.jinja_env.from_string(str(path))
+        return Path(template.render(**self.answers.combined))
 
     @cached_property
     def all_exclusions(self) -> StrSeq:

tests/test_answersfile_templating.py

@@ -0,0 +1,74 @@
+import pytest
+
+import copier
+from copier.user_data import load_answersfile_data
+
+from .helpers import build_file_tree
+
+
+@pytest.fixture(scope="module")
+def template_path(tmp_path_factory) -> str:
+    root = tmp_path_factory.mktemp("template")
+    build_file_tree(
+        {
+            root
+            / "{{ _copier_conf.answers_file }}.jinja": """\
+                # Changes here will be overwritten by Copier
+                {{ _copier_answers|to_nice_yaml }}
+                """,
+            root
+            / "copier.yml": """\
+                _answers_file: ".copier-answers-{{ module_name }}.yml"
+
+                module_name:
+                    type: str
+                """,
+        }
+    )
+    return str(root)
+
+
+@pytest.mark.parametrize("answers_file", [None, ".changed-by-user.yml"])
+def test_answersfile_templating(template_path, tmp_path, answers_file):
+    """
+    Test copier behaves properly when _answers_file contains a template
+
+    Checks that template is resolved successfully and that a subsequent
+    copy that resolves to a different answers file doesn't clobber the
+    old answers file.
+    """
+    copier.copy(
+        template_path,
+        tmp_path,
+        {"module_name": "mymodule"},
+        answers_file=answers_file,
+        defaults=True,
+        overwrite=True,
+    )
+    first_answers_file = (
+        ".copier-answers-mymodule.yml"
+        if answers_file is None
+        else ".changed-by-user.yml"
+    )
+    assert (tmp_path / first_answers_file).exists()
+    answers = load_answersfile_data(tmp_path, first_answers_file)
+    assert answers["module_name"] == "mymodule"
+
+    copier.copy(
+        template_path,
+        tmp_path,
+        {"module_name": "anothermodule"},
+        defaults=True,
+        overwrite=True,
+    )
+
+    # Assert second one created
+    second_answers_file = ".copier-answers-anothermodule.yml"
+    assert (tmp_path / second_answers_file).exists()
+    answers = load_answersfile_data(tmp_path, second_answers_file)
+    assert answers["module_name"] == "anothermodule"
+
+    # Assert first one still exists
+    assert (tmp_path / first_answers_file).exists()
+    answers = load_answersfile_data(tmp_path, first_answers_file)
+    assert answers["module_name"] == "mymodule"