From 122323853d5a17fcd0e2aa7dbb3d4fd50223deb5 Mon Sep 17 00:00:00 2001
From: Brett Smith <brettcsmith@brettcsmith.org>
Date: Tue, 19 Dec 2017 08:33:38 -0500
Subject: [PATCH] importers: Prefer using ChainMaps to copying and updating
 dicts.

---
 import2ledger/importers/_csv.py     | 14 +++++++-------
 import2ledger/importers/nbpy2017.py |  6 ++----
 2 files changed, 9 insertions(+), 11 deletions(-)

diff --git a/import2ledger/importers/_csv.py b/import2ledger/importers/_csv.py
index 5b12d99..754c3fa 100644
--- a/import2ledger/importers/_csv.py
+++ b/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)
diff --git a/import2ledger/importers/nbpy2017.py b/import2ledger/importers/nbpy2017.py
index 9f1bf84..d871fae 100644
--- a/import2ledger/importers/nbpy2017.py
+++ b/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)