feat: allow overriding data file with CLI arguments (#1332)

Users should be able to pass CLI data arguments and use a `--data-file` at the same time.  Let's just take inspiration from the [helm values file](https://helm.sh/docs/chart_template_guide/values_files/) and give highest priority to data values passed at the command line.

Author: mspiegel31

copier/cli.py

@@ -104,7 +104,9 @@ class CopierApp(cli.Application):
 class _Subcommand(cli.Application):
     """Base class for Copier subcommands."""
 
-    data: AnyByStrDict = {}
+    def __init__(self, executable):
+        self.data: AnyByStrDict = {}
+        super().__init__(executable)
 
     answers_file = cli.SwitchAttr(
         ["-a", "--answers-file"],
@@ -158,7 +160,6 @@ class _Subcommand(cli.Application):
         "VARIABLE=VALUE",
         list=True,
         help="Make VARIABLE available as VALUE when rendering the template",
-        excludes=["--data-file"],
     )
     def data_switch(self, values: StrSeq) -> None:
         """Update [data][] with provided values.
@@ -175,7 +176,6 @@ def data_switch(self, values: StrSeq) -> None:
         ["--data-file"],
         cli.ExistingFile,
         help="Load data from a YAML file",
-        excludes=["--data"],
     )
     def data_file_switch(self, path: cli.ExistingFile) -> None:
         """Update [data][] with provided values.
@@ -184,7 +184,12 @@ def data_file_switch(self, path: cli.ExistingFile) -> None:
             path: The path to the YAML file to load.
         """
         with open(path) as f:
-            self.data.update(yaml.safe_load(f))
+            file_updates: AnyByStrDict = yaml.safe_load(f)
+
+        updates_without_cli_overrides = {
+            k: v for k, v in file_updates.items() if k not in self.data
+        }
+        self.data.update(updates_without_cli_overrides)
 
     def _worker(self, src_path: OptStr = None, dst_path: str = ".", **kwargs) -> Worker:
         """

docs/configuring.md

@@ -758,10 +758,6 @@ questions with default answers.
     copier copy -fd 'user_name=Manuel Calavera' template destination
     ```
 
-!!! info
-
-    The `-d, --data` flags are mutually exclusive with the [`--data-file`][data_file] flag.
-
 ### `data_file`
 
 -   Format: `str`
@@ -797,9 +793,16 @@ contains your data.
     copier copy -d 'user_name=Manuel Calavera' -d 'age=7' -d 'height=1.83' template destination
     ```
 
+    if you'd like to override some of the answers in the file, `--data` flags always take
+    precedence
+
+    ```shell
+    copier copy -d 'user_name=Bilbo Baggins' --data-file input.yml template destination
+    ```
+
 !!! info
 
-    The `--data-file` flag is mutually exclusive with the [`-d, --data`][data] flags.
+    command line arguments passed via `--data` will always take precedence over the answers file
 
 ### `envops`
 

tests/test_cli.py

@@ -170,18 +170,20 @@ def test_data_file_parsed_by_question_type(
     assert answers["question"] == "answer"
 
 
-def test_data_and_data_file_mutually_exclusive(
+def test_data_cli_takes_precedence_over_data_file(
     tmp_path_factory: pytest.TempPathFactory,
-    capsys: pytest.CaptureFixture[str],
 ):
     src, dst, local = map(tmp_path_factory.mktemp, ("src", "dst", "local"))
-
     build_file_tree(
         {
             (src / "copier.yml"): (
                 """\
                 question:
                     type: str
+                another_question:
+                    type: str
+                yet_another_question:
+                    type: str
                 """
             ),
             (src / "{{ _copier_conf.answers_file }}.jinja"): (
@@ -190,31 +192,38 @@ def test_data_and_data_file_mutually_exclusive(
                 {{ _copier_answers|to_nice_yaml }}
                 """
             ),
-            (local / "data.yml"): yaml.dump({"question": "does not matter"}),
+            (local / "data.yml"): yaml.dump(
+                {
+                    "question": "does not matter",
+                    "another_question": "another answer!",
+                    "yet_another_question": "sure, one more for you!",
+                }
+            ),
         }
     )
 
+    answer_override = "fourscore and seven years ago"
     run_result = CopierApp.run(
         [
             "copier",
             "copy",
             f"--data-file={local / 'data.yml'}",
-            '--data=question="also does not matter"',
+            f"--data=question={answer_override}",
             str(src),
             str(dst),
             "--quiet",
         ],
         exit=False,
     )
-    assert run_result[1] == 2  # cli failure
-    out, _ = capsys.readouterr()
-    assert any(
-        [
-            "Error: Given --data-file, the following are invalid ['--data']" in out,
-            "Error: Given --data, the following are invalid ['--data-file']" in out,
-        ]
-    )
-    assert not (dst / ".copier-answers.yml").exists()
+    assert run_result[1] == 0
+    actual_answers = yaml.safe_load((dst / ".copier-answers.yml").read_text())
+    expected_answers = {
+        "_src_path": str(src),
+        "question": answer_override,
+        "another_question": "another answer!",
+        "yet_another_question": "sure, one more for you!",
+    }
+    assert actual_answers == expected_answers
 
 
 @pytest.mark.parametrize(