Add Exclude on Update

copier/main.py

@@ -954,6 +954,7 @@ def _apply_update(self) -> None:  # noqa: C901
             # Do a normal update in final destination
             with replace(
                 self,
+                exclude=self.exclude + self.template.exclude_on_update,
                 # Files can change due to the historical diff, and those
                 # changes are not detected in this process, so it's better to
                 # say nothing than lie.
@@ -968,6 +969,7 @@ def _apply_update(self) -> None:  # noqa: C901
                 dst_path=new_copy / subproject_subdir,
                 data=self.answers.combined,  # type: ignore[arg-type]
                 defaults=True,
+                exclude=self.exclude + self.template.exclude_on_update,
                 quiet=True,
                 src_path=self.subproject.template.url,  # type: ignore[union-attr]
             ) as new_worker:

copier/template.py

@@ -328,6 +328,14 @@ def exclude(self) -> tuple[str, ...]:
                 DEFAULT_EXCLUDE if Path(self.subdirectory) == Path(".") else [],
             )
         )
+
+    @cached_property
+    def exclude_on_update(self) -> tuple[str, ...]:
+        """Get update exclusions specified in the template.
+
+        See [exclude_on_update][].
+        """
+        return tuple(self.config_data.get("exclude_on_update"))
 
     @cached_property
     def jinja_extensions(self) -> tuple[str, ...]:

docs/configuring.md

@@ -893,6 +893,23 @@ The CLI option can be passed several times to add several patterns.
         copier copy --exclude '*' --exclude '!file-i-want' ./template ./destination
         ```
 
+### `exclude_on_update`
+
+-   Format: `List[str]`
+-   CLI flags: N/A
+-   Default value: `[]`
+
+Excluded files applied only on update command.
+
+!!! example
+
+    ```yaml title="copier.yml"
+    _exclude_on_update:
+        - "pre-commit.sample"
+        - "myfile.example"
+        - "*.dummy"
+    ```
+
 ### `force`
 
 -   Format: `bool`

tests/demo_exclude_on_update/copier.yml

@@ -0,0 +1,3 @@
+_exclude_on_update:
+  - "do_not_update.sample"
+

tests/test_updatediff.py

@@ -1290,3 +1290,64 @@ def test_update_with_new_file_in_template_and_project_via_migration(
         >>>>>>> after updating
         """
     )
+
+
+
+def test_exclude_on_update(tmp_path_factory: pytest.TempPathFactory) -> None:
+    # Template in v1 has a file with a single line;
+    # in v2 it changes that line.
+    # Meanwhile, downstream project appended contents to the first line.
+    src, dst = map(tmp_path_factory.mktemp, ("src", "dst"))
+    filename = "README.md"
+
+    # First, create the template with an initial file
+    build_file_tree(
+        {
+            (src / filename): "upstream version 1",
+            (src / "copier.yaml"): f"_exclude_on_update: ['{filename}']",
+            (src / "{{_copier_conf.answers_file}}.jinja"): (
+                "{{_copier_answers|to_nice_yaml}}"
+            ),
+        }
+    )
+    with local.cwd(src):
+        git_init("hello template")
+        git("tag", "v1")
+
+    # Generate the project a first time, assert the file exists
+    run_copy(str(src), dst, defaults=True, overwrite=True)
+    assert (dst / filename).exists()
+    assert "_commit: v1" in (dst / ".copier-answers.yml").read_text()
+
+    # Start versioning the generated project
+    with local.cwd(dst):
+        git_init("hello project")
+
+        # After first commit, change the file, commit again
+        Path(filename).write_text("upstream version 1 + downstream")
+        git("commit", "-am", "updated file")
+
+    # Now change the template
+    with local.cwd(src):
+        # Update the file
+        Path(filename).write_text("upstream version 2")
+
+        # Commit the changes
+        git("add", ".", "-A")
+        git("commit", "-m", "change line in file")
+        git("tag", "v2")
+
+    # Finally, update the generated project
+    run_update(dst_path=dst, defaults=True, overwrite=True, conflict="inline")
+    assert "_commit: v2" in (dst / ".copier-answers.yml").read_text()
+
+    # Assert that the file still exists, and was left untouched
+    assert (dst / filename).exists()
+
+    expected_contents = dedent(
+        """\
+        upstream version 1 + downstream
+        """
+    )
+    assert (dst / filename).read_text().splitlines() == expected_contents.splitlines()
+    assert not (dst / f"{filename}.rej").exists()