exp init: create params file by default

Note that this won't add support for tracking the params file,
it'll only create the params file. Some changes on error and
prompts are made.

dvc/dependency/param.py

@@ -1,4 +1,3 @@
-import errno
 import logging
 import os
 import typing
@@ -133,29 +132,17 @@ def status(self):
 
     def validate_filepath(self):
         if not self.exists:
-            raise FileNotFoundError(
-                errno.ENOENT, os.strerror(errno.ENOENT), str(self)
-            )
+            raise MissingParamsFile(f"Parameters file '{self}' does not exist")
         if self.isdir():
-            raise IsADirectoryError(
-                errno.EISDIR, os.strerror(errno.EISDIR), str(self)
+            raise ParamsIsADirectoryError(
+                f"'{self}' is a directory, expected a parameters file"
             )
 
     def read_file(self):
         _, ext = os.path.splitext(self.fs_path)
         loader = LOADERS[ext]
 
-        try:
-            self.validate_filepath()
-        except FileNotFoundError as exc:
-            raise MissingParamsFile(
-                f"Parameters file '{self}' does not exist"
-            ) from exc
-        except IsADirectoryError as exc:
-            raise ParamsIsADirectoryError(
-                f"'{self}' is a directory, expected a parameters file"
-            ) from exc
-
+        self.validate_filepath()
         try:
             return loader(self.fs_path, fs=self.repo.fs)
         except ParseError as exc:

dvc/repo/experiments/init.py

@@ -159,32 +159,28 @@ def validate_prompts(
 ) -> Union[Any, Tuple[Any, str]]:
     from dvc.ui.prompt import InvalidResponse
 
+    msg_format = "[yellow]'{0}' does not exist, the {1} will be created.[/]"
     if key == "params":
-        import errno
-
-        from dvc.dependency.param import ParamsDependency
+        from dvc.dependency.param import (
+            MissingParamsFile,
+            ParamsDependency,
+            ParamsIsADirectoryError,
+        )
 
         assert isinstance(value, str)
-        msg_format = (
-            "[prompt.invalid]'{0}' {1}. "
-            "Please retry with an existing parameters file."
-        )
         try:
             ParamsDependency(None, value, repo=repo).validate_filepath()
-        except (IsADirectoryError, FileNotFoundError) as e:
-            suffices = {
-                errno.EISDIR: "is a directory",
-                errno.ENOENT: "does not exist",
-            }
-            raise InvalidResponse(msg_format.format(value, suffices[e.errno]))
+        except MissingParamsFile:
+            return value, msg_format.format(value, "file")
+        except ParamsIsADirectoryError:
+            raise InvalidResponse(
+                f"[prompt.invalid]'{value}' is a directory. "
+                "Please retry with an existing parameters file."
+            )
     elif key in ("code", "data"):
         if not os.path.exists(value):
             typ = "file" if is_file(value) else "directory"
-            return (
-                value,
-                f"[yellow]'{value}' does not exist, "
-                f"the {typ} will be created. ",
-            )
+            return value, msg_format.format(value, typ)
     return value
 
 
@@ -260,7 +256,6 @@ def init(
     context: Dict[str, str] = {**defaults, **overrides}
     assert "cmd" in context
 
-    params_kv = []
     params = context.get("params")
     if params:
         from dvc.dependency.param import (
@@ -270,18 +265,19 @@ def init(
         )
 
         try:
-            params_d = ParamsDependency(None, params, repo=repo).read_file()
-        except (MissingParamsFile, ParamsIsADirectoryError) as exc:
+            ParamsDependency(None, params, repo=repo).validate_filepath()
+        except ParamsIsADirectoryError as exc:
             raise DvcException(f"{exc}.")  # swallow cause for display
-        params_kv.append({params: list(params_d.keys())})
+        except MissingParamsFile:
+            pass
 
     checkpoint_out = bool(context.get("live"))
     models = context.get("models")
     stage = repo.stage.create(
         name=name,
         cmd=context["cmd"],
         deps=compact([context.get("code"), context.get("data")]),
-        params=params_kv,
+        params=[{params: None}] if params else None,
         metrics_no_cache=compact([context.get("metrics")]),
         plots_no_cache=compact([context.get("plots")]),
         live=context.get("live"),

tests/func/experiments/test_init.py

@@ -38,7 +38,7 @@ def test_init_simple(tmp_dir, scm, dvc, capsys):
                 "deps": ["data", "src"],
                 "metrics": [{"metrics.json": {"cache": False}}],
                 "outs": ["models"],
-                "params": ["foo"],
+                "params": [{"params.yaml": None}],
                 "plots": [{"plots": {"cache": False}}],
             }
         }
@@ -76,6 +76,24 @@ def test_when_stage_force_if_already_exists(tmp_dir, dvc):
     assert d["stages"]["train"]["cmd"] == "true"
 
 
+@pytest.mark.parametrize("interactive", [True, False])
+def test_creates_params_file_by_default(tmp_dir, dvc, interactive, capsys):
+    init(
+        dvc,
+        interactive=interactive,
+        defaults=CmdExperimentsInit.DEFAULTS,
+        overrides={"cmd": "cmd"},
+        stream=io.StringIO(""),
+    )
+
+    assert (tmp_dir / "params.yaml").is_file()
+    assert (tmp_dir / "params.yaml").parse() == {}
+    out, err = capsys.readouterr()
+    assert "Created src, data and params.yaml." in out
+    if interactive:
+        assert "'params.yaml' does not exist, the file will be created." in err
+
+
 def test_with_a_custom_name(tmp_dir, dvc):
     init(dvc, name="custom", overrides={"cmd": "cmd"})
     assert (tmp_dir / "dvc.yaml").parse() == {
@@ -111,7 +129,7 @@ def test_init_interactive_when_no_path_prompts_need_to_be_asked(
         interactive=True,
         defaults=CmdExperimentsInit.DEFAULTS,
         overrides={**CmdExperimentsInit.DEFAULTS, **extra_overrides},
-        stream=inp,  # we still need to confirm
+        stream=inp,
     )
     assert (tmp_dir / "dvc.yaml").parse() == {
         "stages": {
@@ -123,7 +141,7 @@ def test_init_interactive_when_no_path_prompts_need_to_be_asked(
                 # we specify `live` through `overrides`,
                 # so it creates checkpoint-based output.
                 "outs": [{"models": {"checkpoint": True}}],
-                "params": ["foo"],
+                "params": [{"params.yaml": None}],
                 "plots": [{"plots": {"cache": False}}],
             }
         }
@@ -164,9 +182,7 @@ def test_when_params_is_omitted_in_interactive_mode(tmp_dir, scm, dvc):
 def test_init_interactive_params_validation(tmp_dir, dvc, capsys):
     tmp_dir.gen({"data": {"foo": "foo"}})
     (tmp_dir / "params.yaml").dump({"foo": 1})
-    inp = io.StringIO(
-        "python script.py\nscript.py\ndata\nmodels\nparams.json\ndata\n"
-    )
+    inp = io.StringIO("python script.py\nscript.py\ndata\nmodels\ndata\n")
 
     init(
         dvc, stream=inp, interactive=True, defaults=CmdExperimentsInit.DEFAULTS
@@ -179,7 +195,7 @@ def test_init_interactive_params_validation(tmp_dir, dvc, capsys):
                 "deps": ["data", "script.py"],
                 "metrics": [{"metrics.json": {"cache": False}}],
                 "outs": ["models"],
-                "params": ["foo"],
+                "params": [{"params.yaml": None}],
                 "plots": [{"plots": {"cache": False}}],
             }
         }
@@ -189,9 +205,6 @@ def test_init_interactive_params_validation(tmp_dir, dvc, capsys):
 
     out, err = capsys.readouterr()
     assert (
-        "Path to a parameters file [params.yaml, n to omit]: "
-        "'params.json' does not exist. "
-        "Please retry with an existing parameters file.\n"
         "Path to a parameters file [params.yaml, n to omit]: "
         "'data' is a directory. "
         "Please retry with an existing parameters file.\n"
@@ -262,7 +275,7 @@ def test_init_default(tmp_dir, scm, dvc, interactive, overrides, inp, capsys):
                 "deps": ["data", "script.py"],
                 "metrics": [{"metrics.json": {"cache": False}}],
                 "outs": ["models"],
-                "params": ["foo"],
+                "params": [{"params.yaml": None}],
                 "plots": [{"plots": {"cache": False}}],
             }
         }
@@ -347,7 +360,7 @@ def test_init_interactive_live(
                 "deps": ["data", "script.py"],
                 "live": {"dvclive": {"html": True, "summary": True}},
                 "outs": [{"models": {"checkpoint": True}}],
-                "params": ["foo"],
+                "params": [{"params.yaml": None}],
             }
         }
     }
@@ -396,7 +409,7 @@ def test_init_with_type_live_and_models_plots_provided(
                 "live": {"dvclive": {"html": True, "summary": True}},
                 "metrics": [{"m": {"cache": False}}],
                 "outs": [{"models": {"checkpoint": True}}],
-                "params": ["foo"],
+                "params": [{"params.yaml": None}],
                 "plots": [{"p": {"cache": False}}],
             }
         }
@@ -431,7 +444,7 @@ def test_init_with_type_default_and_live_provided(
                 "live": {"live": {"html": True, "summary": True}},
                 "metrics": [{"metrics.json": {"cache": False}}],
                 "outs": [{"models": {"checkpoint": True}}],
-                "params": ["foo"],
+                "params": [{"params.yaml": None}],
                 "plots": [{"plots": {"cache": False}}],
             }
         }