Weather App Tutorial. Part 3 of 5. Management Command

As you might have noticed, this is a continuous tutorial about an app which regularly checks weather and reports how it changes throughout years. Previously I wrote how to start a project and define the models. The interestingness of the app starts now. I will show you how to import weather details from Yahoo! Weather using a management command.

Management commands in Django are those which are recognized by manage.py and django-admin.py, e.g. syncdb, shell, runserver, etc. It's not difficult to inject your own custom commands for your app. We'll need one that will be set as a scheduled task to check the current weather.

To create a management command, you have to create directories management/commands/ inside of your climate_change directory and create empty files __init__.py in each of them.

mkdir -p climate_change/management/commands
touch climate_change/management/__init__.py
touch climate_change/management/commands/__init__.py

Now I'll create a file climate_change/management/commands/check_weather.py which will aggregate the data from the feed. Just looking at the examples of Django core commands, you can find, that management-command modules should have a class Command with an overridden method handle_norargs which will have all the logic of the command. I found an example of importing weather from Yahoo! and will be using it here.

# -*- coding: UTF-8 -*-
import urllib
from xml.dom import minidom
from pprint import pprint
from datetime import datetime

from django.db import models
from django.core.management.base import NoArgsCommand

Location = models.get_model("climate_change", "Location")
WeatherLog = models.get_model("climate_change", "WeatherLog")

SILENT, NORMAL, VERBOSE = 0, 1, 2

WEATHER_URL = 'http://xml.weather.yahoo.com/forecastrss?p=%s&u=c'
WEATHER_NS = 'http://xml.weather.yahoo.com/ns/rss/1.0'

def weather_for_location(location_id):
    # taken from http://developer.yahoo.com/python/python-xml.html
    # and modified a little
    url = WEATHER_URL % location_id
    dom = minidom.parse(urllib.urlopen(url))
    forecasts = []
    for node in dom.getElementsByTagNameNS(WEATHER_NS, 'forecast'):
        forecasts.append({
            'date': node.getAttribute('date'),
            'low': node.getAttribute('low'),
            'high': node.getAttribute('high'),
            'condition': node.getAttribute('text')
        })
    ycondition = dom.getElementsByTagNameNS(WEATHER_NS, 'condition')[0]
    ywind = dom.getElementsByTagNameNS(WEATHER_NS, 'wind')[0]
    yatmosphere = dom.getElementsByTagNameNS(WEATHER_NS, 'atmosphere')[0]
    return {
        'current_condition': ycondition.getAttribute('text'),
        'current_temp': ycondition.getAttribute('temp'),
        'current_humidity': yatmosphere.getAttribute('humidity'),
        'current_visibility': yatmosphere.getAttribute('visibility'),
        'current_wind_speed': ywind.getAttribute('speed'),
        'forecasts': forecasts,
        'title': dom.getElementsByTagName('title')[0].firstChild.data,
        'guid': dom.getElementsByTagName('guid')[0].firstChild.data,
    }            

class Command(NoArgsCommand):
    help = "Aggregates data from weather feed"
    def handle_noargs(self, **options):
        verbosity = int(options.get('verbosity', NORMAL))
        
        created_count = 0
        for l in Location.objects.all():
            weather = weather_for_location(l.location_id)
            if verbosity > NORMAL:
                pprint(weather)
            timestamp_parts = map(int, weather['guid'].split("_")[1:-1])
            timestamp = datetime(*timestamp_parts)
            log, created = WeatherLog.objects.get_or_create(
                 location=l,
                 timestamp=timestamp,
                 defaults={
                    'temperature': weather['current_temp'],
                    'humidity': weather['current_humidity'],
                    'wind_speed': weather['current_wind_speed'],
                    'visibility': weather['current_visibility'],
                    }
                 )
            if created:
                created_count += 1
        if verbosity > NORMAL:
            print "New weather logs: %d" % created_count

Now we can try running the command from the project directory using the following:

python manage.py check_weather --verbosity=2

I get this result:

{'current_condition': u'Mostly Cloudy',
 'current_humidity': u'94',
 'current_temp': u'10',
 'current_visibility': u'9.99',
 'current_wind_speed': u'16.09',
 'forecasts': [{'condition': u'Partly Cloudy',
                'date': u'6 Oct 2009',
                'high': u'19',
                'low': u'14'},
               {'condition': u'PM Light Rain',
                'date': u'7 Oct 2009',
                'high': u'23',
                'low': u'14'}],
 'guid': u'GMXX0008_2009_10_06_10_20_CEST',
 'title': u'Yahoo! Weather - Berlin/Schonefeld, GM'}
New weather logs: 1

To define this task as a cron job running hourly, you can either type

echo 0 * * * * cd /path/to/climate_change_env/ && source bin/activate && cd blog_action_day_2009 && python manage.py check_weather && deactivate > crontab.txt
crontab crontab.txt
del crontab.txt

.. or use django-poormanscron app and set the check_weather command there.

OK. Cool. The next thing to do is the template tag which displays the details of recent weather.