prepare_sector: automatically interpolate in config get() function

doc/release_notes.rst

@@ -7,9 +7,11 @@
 Release Notes
 ##########################################
 
-.. Upcoming Release
+Upcoming Release
-.. ================
+================
-..
+
+* Linearly interpolate missing investment periods in year-dependent
+  configuration options.
 
 PyPSA-Eur 0.10.0 (19th February 2024)
 =====================================

scripts/prepare_sector_network.py

@@ -215,7 +215,32 @@ def get(item, investment_year=None):
     """
     Check whether item depends on investment year.
     """
-    return item[investment_year] if isinstance(item, dict) else item
+    if not isinstance(item, dict):
+        return item
+    elif investment_year in item.keys():
+        return item[investment_year]
+    else:
+        logger.warning(
+            f"Investment key {investment_year} not found in dictionary {item}."
+        )
+        keys = sorted(item.keys())
+        if investment_year < keys[0]:
+            logger.warning(f"Lower than minimum key. Taking minimum key {keys[0]}")
+            return item[keys[0]]
+        elif investment_year > keys[-1]:
+            logger.warning(f"Higher than maximum key. Taking maximum key {keys[0]}")
+            return item[keys[-1]]
+        else:
+            logger.warning(
+                "Interpolate linearly between the next lower and next higher year."
+            )
+            lower_key = max(k for k in keys if k < investment_year)
+            higher_key = min(k for k in keys if k > investment_year)
+            lower = item[lower_key]
+            higher = item[higher_key]
+            return lower + (higher - lower) * (investment_year - lower_key) / (
+                higher_key - lower_key
+            )
 
 
 def co2_emissions_year(