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The App Engine web server captures everything the handler script writes to the standard output stream for the response to the web request. It also captures everything the handler script writes to the standard error stream, and stores it as log data. Log data for your application can be viewed and analyzed using the Administration Console, or downloaded using appcfg.py request_logs.
The App Engine Python runtime environment includes special support for the logging module from the Python standard library to understand logging concepts such as log levels ("debug", "info", "warning", "error", "critical").
import logging from google.appengine.api import users from google.appengine.ext import db user = users.get_current_user() if user: q = db.GqlQuery("SELECT * FROM UserPrefs WHERE user = :1", user) results = q.fetch(2) if len(results) > 1: logging.error("more than one UserPrefs object for user %s", str(user)) if len(results) == 0: logging.debug("creating UserPrefs object for user %s", str(user)) userprefs = UserPrefs(user=user) userprefs.put() else: userprefs = results[0] else: logging.debug("creating dummy UserPrefs for anonymous user")