Raise default if it's an exception

records.py

@@ -4,6 +4,7 @@ import os
 from code import interact
 from datetime import datetime
 from collections import OrderedDict
+from inspect import isclass
 
 import tablib
 from docopt import docopt
@@ -13,6 +14,17 @@ from sqlalchemy.ext.declarative import declarative_base
 DATABASE_URL = os.environ.get('DATABASE_URL')
 
 
+def isexception(obj):
+    """Given an object, return a boolean indicating whether it is an instance
+    or subclass of :py:class:`Exception`.
+    """
+    if isinstance(obj, Exception):
+        return True
+    if isclass(obj) and issubclass(obj, Exception):
+        return True
+    return False
+
+
 class Record(object):
     """A row, from a query, from a database."""
     __slots__ = ('_keys', '_values')
@@ -195,6 +207,8 @@ class RecordCollection(object):
         try:
             record = next(self)
         except StopIteration:
+            if isexception(default):
+                raise default
             return default
 
         # Ensure that we don't have more than one row.

tests/test_records.py

@@ -76,6 +76,16 @@ class TestRecordCollection:
         rows = records.RecordCollection(IdRecord(i) for i in range(3))
         raises(ValueError, rows.one)
 
+    def test_one_raises_default_if_its_an_exception_subclass(self):
+        rows = records.RecordCollection(IdRecord(i) for i in range(1))
+        class Cheese(Exception): pass
+        raises(Cheese, rows.one, Cheese)
+
+    def test_one_raises_default_if_its_an_exception_instance(self):
+        rows = records.RecordCollection(IdRecord(i) for i in range(1))
+        class Cheese(Exception): pass
+        raises(Cheese, rows.one, Cheese('cheddar'))
+
 
 class TestRecord: