Optional dependencies. (#2)

transactron/utils/dependencies.py

@@ -1,7 +1,7 @@
 from collections import defaultdict
 
 from abc import abstractmethod, ABC
-from typing import Any, Generic, TypeVar
+from typing import Any, Generic, Optional, TypeVar
 
 
 __all__ = ["DependencyManager", "DependencyKey", "DependencyContext", "SimpleKey", "ListKey"]
@@ -122,15 +122,28 @@ def get_dependency(self, key: DependencyKey[Any, U]) -> U:
 
         The way dependencies are interpreted is dependent on the key type.
         """
-        if not key.empty_valid and key not in self.dependencies:
+        ret = self.get_optional_dependency(key)
+
+        if ret is None:
             raise KeyError(f"Dependency {key} not provided")
 
-        if key in self.cache:
-            return self.cache[key]
+        return ret
+
+    def get_optional_dependency(self, key: DependencyKey[Any, U]) -> Optional[U]:
+        """Gets the dependency for a key, if it exists.
 
+        If the dependency is gettable, the return value is the same as in
+        `get_dependency`. Otherwise, `None` is returned.
+        """
         if key.lock_on_get:
             self.locked_dependencies.add(key)
 
+        if not key.empty_valid and key not in self.dependencies:
+            return None
+
+        if key in self.cache:
+            return self.cache[key]
+
         val = key.combine(self.dependencies[key])
 
         if key.cache: