check for mapper is not None before testing type

python/lsst/daf/persistence/butler.py

@@ -505,7 +505,7 @@ def _convertV1Args(self, root, mapper, mapperArgs):
             (inputs, outputs) - values to be used for inputs and outputs in Butler.__init__
         """
         # mapper ought to be an importable string or a class object (not a mapper class instance)
-        if not isinstance(mapper, basestring) and not inspect.isclass(mapper):
+        if mapper and not isinstance(mapper, basestring) and not inspect.isclass(mapper):
             err = "mapper ought to be an importable string or a class object (not a mapper class instance)"
             # TBD we might have to handle this. It'll be complicated because of e.g. outputRoot & calibRoot
             self.log.warn(err)

tests/testButler.py

@@ -25,15 +25,29 @@
 import unittest
 import lsst.daf.persistence as dp
 import lsst.utils.tests
+import os
+import shutil
+
+ROOT = os.path.abspath(os.path.dirname(__file__))
 
 
 def setup_module(module):
     lsst.utils.tests.init()
 
 
+class TestMapper(dp.Mapper):
+    pass
+
+
 class ButlerTest(unittest.TestCase):
     """Test case for basic Butler operations."""
 
+    testDir = os.path.join(ROOT, 'ButlerTest')
+
+    def tearDown(self):
+        if os.path.exists(self.testDir):
+            shutil.rmtree(self.testDir)
+
     def testV1withV2InitApiRaises(self):
         """Test that a RuntimeError is raised when Butler v1 init api (root,
         mapper, mapperArgs**) is used with Butler v2 init api
@@ -51,6 +65,13 @@ def testV1withV2InitApiRaises(self):
         with self.assertRaises(RuntimeError):
             dp.Butler(inputs='foo/bar', outputs='foo/baz')
 
+    def testV1RepoWithRootOnly(self):
+        repoDir = os.path.join(self.testDir, 'repo')
+        os.makedirs(repoDir)
+        with open(os.path.join(repoDir, '_mapper'), 'w') as f:
+            f.write('__main__.TestMapper')
+        butler = dp.Butler(repoDir)
+
 
 class MemoryTester(lsst.utils.tests.MemoryTestCase):
     pass