Skip undefined dimensions

src/imas2xarray/_io.py

@@ -238,7 +238,8 @@ def get_variables(
         """Get variables from data set.
 
         This function looks up the data location from the
-        `imas2xarray.var_lookup` table, and returns
+        `imas2xarray.var_lookup` table, and returns an xarray dataset.
+        Variable dimensions are automatically retrieved if available.
 
         Parameters
         ----------
@@ -263,8 +264,9 @@ def get_variables(
         """
         var_models = var_lookup.lookup(variables)
 
+        # Attempt to automatically load associated dimensions
         dims = {dim for variable in var_models for dim in variable.dims}
-        var_models |= var_lookup.lookup(dims)
+        var_models |= var_lookup.lookup(dims, skip_missing=True)
 
         for var in var_models:
             if var.ids != ids:

src/imas2xarray/_lookup.py

@@ -79,20 +79,37 @@ def groupby_ids(self) -> dict[Hashable, list[IDSVariableModel]]:
         grouped_ids_vars = groupby(ids_vars, keyfunc=lambda var: var.ids)
         return grouped_ids_vars
 
-    def lookup(self, variables: Collection[(str | IDSVariableModel)]) -> set[IDSVariableModel]:
+    def lookup(
+        self, variables: Collection[(str | IDSVariableModel)], skip_missing: bool = False
+    ) -> set[IDSVariableModel]:
         """Helper function to look up a bunch of variables.
 
-        If str, look up the variable from the `var_lookup`. Else, check if
-        the variable is an `IDSVariableModel`.
+        Parameters
+        ----------
+        variables : Collection[(str | IDSVariableModel)]
+            List of variables to load. If str, look up the variable from the `var_lookup`.
+            Else, ensure the variable is an `IDSVariableModel`.
+        skip_missing : bool
+            Skip missing variables
+
+        Returns
+        -------
+        variables : set[IDSVariableModel]
+            Collection of variable models
         """
         var_models = set()
 
         for var in variables:
             if isinstance(var, str):
-                if var.endswith(ERROR_SUFFIX):
-                    var = self.error_upper(var)
-                else:
-                    var = self[var]
+                try:
+                    if var.endswith(ERROR_SUFFIX):
+                        var = self.error_upper(var)
+                    else:
+                        var = self[var]
+                except KeyError:
+                    if skip_missing:
+                        continue
+                    raise
             if not isinstance(var, IDSVariableModel):
                 raise ValueError(f'Cannot lookup variable with type {type(var)}')