importers.csvbase: Allow finalize() to drop the current transaction

Returning None from finalize() results in the current transaction
being discarded and in the source row to do not participate in the
determination of the balances.

beangulp/importers/csvbase.py

@@ -318,6 +318,8 @@ def extract(self, filepath, existing):
 
             # Apply user processing to the transaction.
             txn = self.finalize(txn, row)
+            if txn is None:
+                continue
 
             # Add the transaction to the output list.
             entries.append(txn)
@@ -366,12 +368,16 @@ def metadata(self, filepath, lineno, row):
     def finalize(self, txn, row):
         """Post process the transaction.
 
+        Returning None results in the transaction being discarded and
+        in source row to do not contribute to the determination of the
+        balances.
+
         Args:
           txn: The just build Transaction object.
           row: The data row being processed.
 
         Returns:
-          A potentially extended or modified Transaction object.
+          A potentially extended or modified Transaction object or None.
 
         """
         return txn

beangulp/importers/csvbase_test.py

@@ -529,3 +529,24 @@ def finalize(self, txn, row):
           Assets:CSV  -1.00 EUR
           Expenses:Testing  1.00 EUR
         """)
+
+    @docfile
+    def test_extract_finalize_remove(self, filename):
+        """\
+        2021-05-17, Test, -1.00, Testing
+        2021-05-18, Drop, -2.00, Testing
+        """
+        class CSVImporter(Base):
+            date = Date(0)
+            narration = Column(1)
+            amount = Amount(2)
+            names = False
+
+            def finalize(self, txn, row):
+                if txn.narration == 'Drop':
+                    return None
+                return txn
+
+        importer = CSVImporter('Assets:CSV', 'EUR')
+        entries = importer.extract(filename, [])
+        self.assertEqual(len(entries), 1)