Add keyCheck user callback function to ConfigDictField

python/lsst/pex/config/configDictField.py

@@ -77,6 +77,11 @@ def __setitem__(self, k, x, at=None, label="setitem", setHistory=True):
             )
             raise FieldValidationError(self._field, self._config, msg)
 
+        # validate key using keycheck
+        if self._field.keyCheck is not None and not self._field.keyCheck(k):
+            msg = f"Key {k!r} is not a valid key"
+            raise FieldValidationError(self._field, self._config, msg)
+
         if at is None:
             at = getCallStack()
         name = _joinNamePath(self._config._name, self._field.name, k)
@@ -127,6 +132,8 @@ class ConfigDictField(DictField):
         Default is `True`.
     dictCheck : `~collections.abc.Callable` or `None`, optional
         Callable to check a dict.
+    keyCheck : `~collections.abc.Callable` or `None`, optional
+        Callable to check a key.
     itemCheck : `~collections.abc.Callable` or `None`, optional
         Callable to check an item.
     deprecated : None or `str`, optional
@@ -140,7 +147,8 @@ class ConfigDictField(DictField):
 
         - ``keytype`` or ``itemtype`` arguments are not supported types
           (members of `ConfigDictField.supportedTypes`.
-        - ``dictCheck`` or ``itemCheck`` is not a callable function.
+        - ``dictCheck``, ``keyCheck`` or ``itemCheck`` is not a callable
+          function.
 
     See Also
     --------
@@ -172,6 +180,7 @@ def __init__(
         default=None,
         optional=False,
         dictCheck=None,
+        keyCheck=None,
         itemCheck=None,
         deprecated=None,
     ):
@@ -191,12 +200,15 @@ def __init__(
             raise ValueError(f"'itemtype' {_typeStr(itemtype)} is not a supported type")
         if dictCheck is not None and not hasattr(dictCheck, "__call__"):
             raise ValueError("'dictCheck' must be callable")
+        if keyCheck is not None and not hasattr(keyCheck, "__call__"):
+            raise ValueError("'keyCheck' must be callable")
         if itemCheck is not None and not hasattr(itemCheck, "__call__"):
             raise ValueError("'itemCheck' must be callable")
 
         self.keytype = keytype
         self.itemtype = itemtype
         self.dictCheck = dictCheck
+        self.keyCheck = keyCheck
         self.itemCheck = itemCheck
 
     def rename(self, instance):
@@ -210,6 +222,9 @@ def validate(self, instance):
         value = self.__get__(instance)
         if value is not None:
             for k in value:
+                if self.keyCheck is not None and not self.keyCheck(k):
+                    msg = f"Key {k!r} is not a valid key"
+                    raise FieldValidationError(self, instance, msg)
                 item = value[k]
                 item.validate()
                 if self.itemCheck is not None and not self.itemCheck(item):