And here's the test with did the good work leading to the previous commit.

test/test_catalog.py

@@ -0,0 +1,18 @@
+import unittest
+
+from hypothesis import given, example
+from hypothesis.specifiers import dictionary
+from segpy.catalog import CatalogBuilder
+
+
+class TestCatalogBuilder(unittest.TestCase):
+
+    @given(dictionary(int, int))
+    def test_constructor(self, mapping):
+        builder = CatalogBuilder(mapping)
+        catalog = builder.create()
+        shared_items = set(mapping.items()) & set(catalog.items())
+        self.assertEqual(len(shared_items), len(mapping))
+
+
+