Ensure caching works if there is no pre-existing data

Author: PeterJCLaw

scripts/element_tracking.py

@@ -52,8 +52,12 @@ class ElementsInProgress:
     mappings of tasks that are known to be equivalent to those in this repo.
     """
 
-    def __init__(self, known_elements: Optional[MutableMapping[str, int]]) -> None:
-        self.known_elements = known_elements or {}
+    def __init__(self, known_elements: MutableMapping[str, int]) -> None:
+        # Note: it's important that we use the mapping the user passes in so
+        # that they can pass an object that will keep track of the elements that
+        # have been processed (either a `shelve` or the mapping yielded by an
+        # `ElementsCache` instance work well).
+        self.known_elements = known_elements
         self._current: Set[str] = set()
 
     def get(self, key: str) -> Optional[int]:

scripts/import.py

@@ -3,7 +3,7 @@
 import argparse
 import contextlib
 from pathlib import Path
-from typing import TYPE_CHECKING, Callable, MutableMapping, Optional
+from typing import TYPE_CHECKING, Callable, MutableMapping
 
 import yaml
 from element_tracking import ElementsCache, ElementsInProgress
@@ -86,7 +86,7 @@ def add(
     backend: 'Backend',
     year: str,
     *,
-    known_elements: Optional[MutableMapping[str, int]] = None,
+    known_elements: MutableMapping[str, int],
 ) -> int:
     """
     Add 'element' into the task tracker, along with all its dependencies.