Honor the PYTHON_KEYRING_BACKEND environment variable to allow an env…

…ironment to select a particular keyring. Fixes #340.

CHANGES.rst

@@ -4,6 +4,10 @@
 * #340: Add the Null keyring, disabled by default.
 * #340: Added ``--disable`` option to command-line
   interface.
+* #340: Now honor a ``PYTHON_KEYRING_BACKEND``
+  environment variable to select a backend. Environments
+  may set to ``keyring.backends.null.Keyring`` to disable
+  keyring.
 
 15.0.0
 ------

keyring/core.py

@@ -84,7 +84,8 @@ def init_backend(limit=None):
     keyrings = filter(limit, backend.get_all_keyring())
 
     set_keyring(
-        load_config()
+        load_env()
+        or load_config()
         or max(keyrings, default=fail.Keyring, key=by_priority)
     )
 
@@ -128,6 +129,14 @@ def load_keyring(keyring_name):
     return class_()
 
 
+def load_env():
+    """Load a keyring configured in the environment variable."""
+    try:
+        return load_keyring(os.environ['PYTHON_KEYRING_BACKEND'])
+    except KeyError:
+        pass
+
+
 def load_config():
     """Load a keyring using the config file in the config root."""