Use different singleton pattern to support inheritance

The previous singleton pattern doesn't work when a class is inherited from the GeoIP class
and causes creation of a new GeoIP object every time GeoIP is accessed. In addition add
locks to protect from multiple instance creation from different threads.

pygeoip/__init__.py

@@ -51,33 +51,36 @@ MEMORY_CACHE = const.MEMORY_CACHE
 ENCODING = const.ENCODING
 
 
+class GeoIPError(Exception):
+    pass
+
+
 class _GeoIPMetaclass(type):
-    def __new__(cls, *args, **kwargs):
+    _instances = {}
+    _instance_lock = Lock()
+
+    def __call__(cls, *args, **kwargs):
         """ Singleton method to gets an instance without reparsing
         the database, the filename is being used as cache key.
         """
-        if not hasattr(cls, '_instances'):
-            cls._instances = {}
-
         if len(args) > 0:
             filename = args[0]
         elif 'filename' in kwargs:
             filename = kwargs['filename']
+        else:
+            return None
 
+        cls._instance_lock.acquire()
         if filename not in cls._instances:
-            cls._instances[filename] = type.__new__(cls, *args, **kwargs)
+            cls._instances[filename] = super(_GeoIPMetaclass, cls).__call__(*args, **kwargs)
+        cls._instance_lock.release()
 
         return cls._instances[filename]
 
 
-_GeoIPBase = _GeoIPMetaclass('GeoIPBase', (object,), {})
-
-
-class GeoIPError(Exception):
-    pass
-
+class GeoIP(object):
+    __metaclass__ = _GeoIPMetaclass
 
-class GeoIP(_GeoIPBase):
     def __init__(self, filename, flags=0):
         """
         Initialize the class.