OmegaConf.missing_keys to get a set of missing keys

docs/source/usage.rst

@@ -782,6 +782,24 @@ and ``OmegaConf.is_list(cfg)`` is equivalent to ``isinstance(cfg, ListConfig)``.
     >>> assert not OmegaConf.is_dict(l)
 
 
+OmegaConf.missing_keys
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Gives a set of missing keys represented in a dotlist style.
+This utility function can be used at the end of creating configuration, after merging sources and so on,
+to check for remaining mandatory fields and prompt a proper error message.
+
+.. doctest::
+
+    >>> OmegaConf.missing_keys({
+    ...     "foo": {"bar": "???"},
+    ...     "missing": "???",
+    ...      "list": ["a", None, "???"]
+    ... })
+    {'missing', 'list.2', 'foo.bar'}
+
+A plain dict, list or any valid input for `OmegaConf.create` is also acceptable - the function would raise a `ValueError` exception on wrong input.
+
+
 Debugger integration
 --------------------
 

news/720.feature

@@ -0,0 +1 @@
+New `OmegaConf.missing_keys` method that enables to get a config's set of missing keys represented in a dotlist style.

omegaconf/omegaconf.py

@@ -18,6 +18,7 @@
     Generator,
     List,
     Optional,
+    Set,
     Tuple,
     Type,
     Union,
@@ -785,6 +786,37 @@ def resolve(cfg: Container) -> None:
             )
         omegaconf._impl._resolve(cfg)
 
+    @staticmethod
+    def missing_keys(cfg: Union[Container, Any]) -> Set[str]:
+        """
+        Returns a set of missing keys flatten in a dotlist style.
+        :param cfg: An `OmegaConf.Container`,
+                    or convertible object via `OmegaConf.create` (dict, list, ...).
+        :return: set of strings of the missing keys.
+        :raises ValueError: On input not representing a config.
+        """
+        if not isinstance(cfg, Container):
+            try:
+                cfg = OmegaConf.create(cfg)  # type: ignore
+            except ValidationError:
+                raise ValueError(f'Could not create a config out of {cfg}')
+        missings = set()
+
+        def gather(_cfg: Container, prefix: str = "") -> None:
+            if isinstance(_cfg, ListConfig):
+                itr = range(len(_cfg))
+            else:
+                itr = _cfg
+
+            for key in itr:
+                if OmegaConf.is_missing(_cfg, key):
+                    missings.add(f"{prefix}{key}")
+                elif OmegaConf.is_config(_cfg[key]):
+                    gather(_cfg[key], prefix=f"{prefix}{key}.")
+
+        gather(cfg)
+        return missings
+
     # === private === #
 
     @staticmethod
@@ -907,7 +939,6 @@ def flag_override(
     names: Union[List[str], str],
     values: Union[List[Optional[bool]], Optional[bool]],
 ) -> Generator[Node, None, None]:
-
     if isinstance(names, str):
         names = [names]
     if values is None or isinstance(values, bool):