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+.. _example_handler:
+
+Fundamentals
+================
+
+This basic example shows how to create simple python module which will pass on the requests to the iterator.
+
+How to enable python module
+----------------------------
+If you look into unbound configuration file, you can find the option `module-config` which specifies the names and the order of modules to be used.
+Example configuration::
+
+	module-config: "validator python iterator"
+
+As soon as the DNS query arrives, Unbound calls modules starting from leftmost - the validator *(it is the first module on the list)*.
+The validator does not know the answer *(it can only validate)*, thus it will pass on the event to the next module.
+Next module is python which can
+
+	a) generate answer *(response)*
+		When python module generates the response unbound calls validator. Validator grabs the answer and determines the security flag.
+
+	b) pass on the event to the iterator.
+		When iterator resolves the query, Unbound informs python module (event :data:`module_event_moddone`). In the end, when the python module is done, validator is called.
+
+Note that the python module is called with :data:`module_event_pass` event, because new DNS event was already handled by validator.
+
+Another situation occurs when we use the following configuration::
+
+	module-config: "python validator iterator"
+
+Python module is the first module here, so it's invoked with :data:`module_event_new` event *(new query)*.
+
+On Python module initialization, module loads script from `python-script` option::
+
+	python-script: "/unbound/test/ubmodule.py"
+
+Simple python module step by step
+---------------------------------
+
+Script file must contain four compulsory functions:
+
+.. function:: init(id, cfg)
+
+   Initialize module internals, like database etc.
+   Called just once on module load.
+
+   :param id: module identifier (integer)
+   :param cfg: :class:`config_file` configuration structure
+
+::
+
+   def init(id, cfg):
+      log_info("pythonmod: init called, module id is %d port: %d script: %s" % (id, cfg.port, cfg.python_script))
+      return True
+
+
+.. function:: deinit(id)
+
+   Deinitialize module internals.
+   Called just once on module unload.
+
+   :param id: module identifier (integer)
+
+::
+
+   def deinit(id):
+      log_info("pythonmod: deinit called, module id is %d" % id)
+      return True
+
+
+.. function:: inform_super(id, qstate, superqstate, qdata)
+
+   Inform super querystate about the results from this subquerystate.
+   Is called when the querystate is finished.
+
+   :param id: module identifier (integer)
+   :param qstate: :class:`module_qstate` Query state
+   :param superqstate: :class:`pythonmod_qstate` Mesh state
+   :param qdata: :class:`query_info` Query data
+
+::
+
+   def inform_super(id, qstate, superqstate, qdata):
+      return True
+
+
+
+.. function:: operate(id, event, qstate, qdata)
+
+   Perform action on pending query. Accepts a new query, or work on pending query.
+
+   You have to set qstate.ext_state on exit.
+   The state informs unbound about result and controls the following states.
+
+   :param id: module identifier (integer)
+   :param qstate: :class:`module_qstate` query state structure
+   :param qdata: :class:`query_info` per query data, here you can store your own data
+
+::
+
+   def operate(id, event, qstate, qdata):
+      log_info("pythonmod: operate called, id: %d, event:%s" % (id, strmodulevent(event)))
+      if event == MODULE_EVENT_NEW:
+         qstate.ext_state[id] = MODULE_WAIT_MODULE 
+         return True
+
+      if event == MODULE_EVENT_MODDONE:
+         qstate.ext_state[id] = MODULE_FINISHED 
+         return True
+
+      if event == MODULE_EVENT_PASS:
+         qstate.ext_state[id] = MODULE_ERROR 
+         return True
+
+      log_err("pythonmod: BAD event")
+      qstate.ext_state[id] = MODULE_ERROR
+      return True
+
+
+Complete source code
+--------------------
+
+..	literalinclude:: example0-1.py
+	:language: python
+
+As you can see, the source code is much more flexible in contrast to C modules. 
+Moreover, compulsory functions called on appropriate module events allows to handle almost
+anything from web control to query analysis.
+