Raise error on invalid use of runpath placeholders

src/ert/config/model_config.py

@@ -54,6 +54,16 @@ def __init__(
         self.history_source = history_source
         self.jobname_format_string = _replace_runpath_format(jobname_format_string)
         self.eclbase_format_string = _replace_runpath_format(eclbase_format_string)
+
+        count_placeholders = (
+            runpath_format_string.count("%d") if runpath_format_string else 0
+        )
+
+        if not count_placeholders in (0, 2):
+            raise ConfigValidationError(
+                f"RUNPATH provided is invalid: `{runpath_format_string}`. Valid example `{DEFAULT_RUNPATH}`"
+            )
+
         self.runpath_format_string = _replace_runpath_format(runpath_format_string)
 
         if self.runpath_format_string is not None and not any(

tests/unit_tests/test_run_path_creation.py

@@ -5,7 +5,7 @@
 
 import pytest
 
-from ert.config import ErtConfig
+from ert.config import ConfigValidationError, ErtConfig
 from ert.enkf_main import create_run_path, ensemble_context, sample_prior
 from ert.run_context import RunContext
 from ert.runpaths import Runpaths
@@ -469,3 +469,35 @@ def test_num_cpu_subst(monkeypatch, tmp_path, append, numcpu, storage):
 
     with open("simulations/realization-0/iter-0/jobs.json", encoding="utf-8") as f:
         assert f'"argList": ["{numcpu}"]' in f.read()
+
+
+@pytest.mark.parametrize(
+    "run_path, expected_raise",
+    [
+        ("simulations/realizations-<IENS>/iter-<ITER>", False),
+        ("simulations/realizations-<IENS>/iter-%d", True),
+        ("simulations/realizations-%d", True),
+        ("simulations/realizations-%d/iter-%d", False),
+        ("simulations/realizations-%d/iter-%d/more-%d", True),
+    ],
+)
+@pytest.mark.usefixtures("use_tmpdir")
+def test_that_runpath_shall_contain_zero_or_two_substitutions(run_path, expected_raise):
+    """
+    This checks that RUNPATH does not include too many or few substitution placeholders
+    """
+    config_text = dedent(
+        """
+        NUM_REALIZATIONS 2
+        """
+    )
+    Path("config.ert").write_text(config_text + f"RUNPATH {run_path}", encoding="utf-8")
+
+    if expected_raise:
+        with pytest.raises(
+            ConfigValidationError,
+            match=f"RUNPATH provided is invalid: `.*{run_path}`.",
+        ):
+            _ = ErtConfig.from_file("config.ert")
+    else:
+        _ = ErtConfig.from_file("config.ert")