Fix #11491. When dbm.open was called with a file which already exists and

the "flag" argument is "n", dbm.error was being raised. As documented,
dbm.open(...,flag='n') will now "Always create a new, empty database,
open for reading and writing", regardless of a previous file existing.

2 files changed, 11 insertions, 3 deletions

diff --git a/Lib/dbm/__init__.py b/Lib/dbm/__init__.py
index c224847..99c1637 100644
--- a/Lib/dbm/__init__.py
+++ b/Lib/dbm/__init__.py
@@ -68,10 +68,10 @@ def open(file, flag = 'r', mode = 0o666):
         if not _defaultmod:
             raise ImportError("no dbm clone found; tried %s" % _names)
 
-    # guess the type of an existing database
-    result = whichdb(file)
+    # guess the type of an existing database, if not creating a new one
+    result = whichdb(file) if 'n' not in flag else None
     if result is None:
-        # db doesn't exist
+        # db doesn't exist or 'n' flag was specified to create a new db
         if 'c' in flag or 'n' in flag:
             # file doesn't exist and the new flag was used so use default type
             mod = _defaultmod
diff --git a/Lib/test/test_dbm.py b/Lib/test/test_dbm.py
index 818be45..ce48cfd 100644
--- a/Lib/test/test_dbm.py
+++ b/Lib/test/test_dbm.py
@@ -70,6 +70,14 @@ class AnyDBMTestCase(unittest.TestCase):
         self.read_helper(f)
         f.close()
 
+    def test_anydbm_creation_n_file_exists_with_invalid_contents(self):
+        with open(_fname, "w") as w:
+            pass # create an empty file
+
+        f = dbm.open(_fname, 'n')
+        self.addCleanup(f.close)
+        self.assertEqual(len(f), 0)
+
     def test_anydbm_modification(self):
         self.init_db()
         f = dbm.open(_fname, 'c')