Redirect logging output using custom logging handler

Question

Hi Guys,

I'm using a module in my python app that writes a lot a of messages using the logging module. Initially I was using this in a console application and it was pretty easy to get the logging output to display on the console using a console handler. Now I've developed a GUI version of my app using wxPython and I'd like to display all the logging output to a custom control — a multi-line textCtrl. Is there a way i could create a custom logging handler so i can redirect all the logging output there and display the logging messages wherever/however I want — in this case, a wxPython app.

Thanks

Answer 1

You will need to create a custom logging.Handler and add it to your logging.Logger.

From the documentation:

Handler objects are responsible for dispatching the appropriate log messages (based on the log messages’ severity) to the handler’s specified destination. Logger objects can add zero or more handler objects to themselves with an addHandler() method. As an example scenario, an application may want to send all log messages to a log file, all log messages of error or higher to stdout, and all messages of critical to an email address. This scenario requires three individual handlers where each handler is responsible for sending messages of a specific severity to a specific location.

See http://docs.python.org/library/logging.html#handler-objects for the Handler API.

In particular, it is the Handler.emit(record) method that you can implement to specify the destination of the output. Presumably, you would implement this to call TextCtrl.AppendText.

Answer 2

Create Handler

import wx
import wx.lib.newevent

import logging

# create event type
wxLogEvent, EVT_WX_LOG_EVENT = wx.lib.newevent.NewEvent()


class wxLogHandler(logging.Handler):
    """
    A handler class which sends log strings to a wx object
    """
    def __init__(self, wxDest=None):
        """
        Initialize the handler
        @param wxDest: the destination object to post the event to 
        @type wxDest: wx.Window
        """
        logging.Handler.__init__(self)
        self.wxDest = wxDest
        self.level = logging.DEBUG

    def flush(self):
        """
        does nothing for this handler
        """


    def emit(self, record):
        """
        Emit a record.

        """
        try:
            msg = self.format(record)
            evt = wxLogEvent(message=msg,levelname=record.levelname)            
            wx.PostEvent(self.wxDest,evt)
        except (KeyboardInterrupt, SystemExit):
            raise
        except:
            self.handleError(record)

Then in your control

self.Bind(EVT_WX_LOG_EVENT, self.onLogEvent)

def onLogEvent(self,event):
    '''
    Add event.message to text window
    '''
    msg = event.message.strip("\r")+"\n"
    self.logwindow.AppendText(msg) # or whatevery
    event.Skip()

Answer 3

Here's a simple working example:

import logging
import random
import sys
import wx

logger = logging.getLogger(__name__)

class WxTextCtrlHandler(logging.Handler):
    def __init__(self, ctrl):
        logging.Handler.__init__(self)
        self.ctrl = ctrl

    def emit(self, record):
        s = self.format(record) + '\n'
        wx.CallAfter(self.ctrl.WriteText, s)

LEVELS = [
    logging.DEBUG,
    logging.INFO,
    logging.WARNING,
    logging.ERROR,
    logging.CRITICAL
]

class Frame(wx.Frame):

    def __init__(self):
        TITLE = "wxPython Logging To A Control"
        wx.Frame.__init__(self, None, wx.ID_ANY, TITLE)

        panel = wx.Panel(self, wx.ID_ANY)
        log = wx.TextCtrl(panel, wx.ID_ANY, size=(300,100),
                          style = wx.TE_MULTILINE|wx.TE_READONLY|wx.HSCROLL)
        btn = wx.Button(panel, wx.ID_ANY, 'Log something!')
        self.Bind(wx.EVT_BUTTON, self.onButton, btn)

        sizer = wx.BoxSizer(wx.VERTICAL)
        sizer.Add(log, 1, wx.ALL|wx.EXPAND, 5)
        sizer.Add(btn, 0, wx.ALL|wx.CENTER, 5)
        panel.SetSizer(sizer)
        handler = WxTextCtrlHandler(log)
        logger.addHandler(handler)
        FORMAT = "%(asctime)s %(levelname)s %(message)s"
        handler.setFormatter(logging.Formatter(FORMAT))
        logger.setLevel(logging.DEBUG)

    def onButton(self, event):
        logger.log(random.choice(LEVELS), "More? click again!")

if __name__ == "__main__":
    app = wx.PySimpleApp()
    frame = Frame().Show()
    app.MainLoop()

Screenshot:

Update: As iondiode points out, this simple script may have problems if there are multiple threads in your app, all logging via such a handler; ideally only a UI thread should update the UI. You can use the suggested approach of logging the event by using a custom event, as per his answer.