importers: Prefer using ChainMaps to copying and updating dicts.

import2ledger/importers/_csv.py

@@ -1,3 +1,4 @@
+import collections
 import csv
 
 class CSVImporterBase:
@@ -27,16 +28,15 @@ class CSVImporterBase:
 
     def __init__(self, input_file):
         self.in_csv = csv.DictReader(input_file)
-        self.entry_seed = self.ENTRY_SEED.copy()
+        self.entry_seed = {}
 
     def __iter__(self):
         for row in self.in_csv:
             row_data = self._read_row(row)
             if row_data is not None:
-                retval = self.entry_seed.copy()
-                retval.update(
-                    (entry_key, row[row_key])
+                copied_fields = {
+                    entry_key: row[row_key]
                     for row_key, entry_key in self.COPIED_FIELDS.items()
-                )
-                retval.update(row_data)
-                yield retval
+                }
+                yield collections.ChainMap(
+                    row_data, copied_fields, self.entry_seed, self.ENTRY_SEED)

import2ledger/importers/nbpy2017.py

@@ -1,3 +1,4 @@
+import collections
 import decimal
 import functools
 
@@ -110,10 +111,7 @@ class Invoice2017:
             self.actions.append(action)
 
     def __iter__(self):
-        for action in self.actions:
-            data = self.base_data.copy()
-            data.update(action)
-            yield data
+        return (collections.ChainMap(act, self.base_data) for act in self.actions)
 
 
 @functools.lru_cache(5)