Wednesday, September 9, 2015

Creating events in Google Calendar from Python

NOTE: The code covered in this blogpost is also available in a video walkthrough here.

UPDATE (Jan 2016): Tweaked the code to support oauth2client.tools.run_flow() which deprecates oauth2client.tools.run(). You can read more about that change and migration steps here.

Introduction

So far in this series of blogposts covering authorized Google APIs, we've used Python code to access Google Drive and Gmail. Today, we're going to demonstrate the Google Calendar API. While Google Calendar, and calendaring in general, have been around for a long time and are fairly stable, it's somewhat of a mystery as to why so few developers create good calendar integrations, whether it be with Google Calendar, or other systems. We'll try to show it isn't necessarily difficult and hopefully motivate some of you out there to add a calendaring feature in your next mobile or web app.

Earlier posts demonstrated the structure and "how-to" use Google APIs in general, so more recent posts, including this one, focus on solutions and apps, and use of specific APIs. Once you review the earlier material, you're ready to start with authorization scopes then see how to use the API itself.

    Google Calendar API Scopes

    Below are the Google Calendar API scopes of authorization. There are only a pair (at the time of this writing): read-only and read/write. As usual, use the most restrictive scope you possibly can yet still allowing your app to do its work. This makes your app more secure and may prevent inadvertently going over any quotas, or accessing, destroying, or corrupting data. Also, users are less hesitant to install your app if it asks only for more restricted access to their calendars. However, it's likely that in order to really use the API to its fullest, you will probably have to ask for read-write so that you can add, update, or delete events in their calendars.
    • 'https://www.googleapis.com/auth/calendar.readonly' — Read-only access to calendar
    • 'https://www.googleapis.com/auth/calendar' — Read/write access to calendar

    Using the Google Calendar API

    We're going to create a sample Python script that inserts a new event into your Google Calendar. Since this requires modifying your calendar, you need the read/write scope above. The API name is 'calendar' which is currently on version 3, so here's the call to apiclient.discovery.build() you'll use:

    CAL = build('calendar', 'v3', http=creds.authorize(Http()))

    Note that all lines of code above that is predominantly boilerplate (that was explained in earlier posts and videos). Anyway, we've got an established service endpoint with build(), we need to come up with the data to create a calendar event with, at the very least, an event name plus start and end times.

    Timezone required (and in a specific format)

    The API requires a GMT offset, the number of hours your timezone is away from Coordinated Universal Time (UTC, more commonly known as GMT). The format is +/-HH:MM away from UTC. For example, Pacific Daylight Time (PDT, also known as Mountain Standard Time, or MST), is "-07:00," or seven hours behind UTC while Nepal Standard Time (NST [or NPT to avoid confusion with Newfoundland Standard Time]), is "+05:45," or five hours and forty-five minutes ahead of UTC. Also, the timezone must be in RFC 3339 format, which implements the specifications of ISO 8601 for the Internet. Timestamps look like the following in the required format: "YYYY-MM-DDTHH:MM:SS±HH:MM". For example, September 15, 2015 at 7 PM PDT is represented by this string: "2015-09-15T19:00:00-07:00".

    The sample script uses the PDT timezone, so we set the GMT_OFF variable to "-07:00". The EVENT body will hold the event name, and start and end times suffixed with the GMT offset:
    GMT_OFF = '-07:00'    # PDT/MST/GMT-7
    EVENT = {
        'summary': 'Dinner with friends',
        'start':   {'dateTime': '2015-09-15T19:00:00%s' % GMT_OFF},
        'end':     {'dateTime': '2015-09-15T22:00:00%s' % GMT_OFF},
    }
    Use the insert() method of the events() service to add the event. As expected, one required parameter is the ID of the calendar to insert the event into. A special value of 'primary' has been set aside for the currently authenticated user. The other required parameter is the event body. In our request, we also ask the Calendar API to send email notifications to the guests, and that's done by passing in the sendNotifications flag with a True value. Our call to the API looks like this:
    e = CAL.events().insert(calendarId='primary',
        sendNotifications=True, body=EVENT).execute()
    The one remaining thing is to confirm that the calendar event was created successfully. We do that by checking the return value — it should be an Event object with all the details we passed in a moment ago:
    print('''*** %r event added:
        Start: %s
        End:   %s''' % (e['summary'].encode('utf-8'),
            e['start']['dateTime'], e['end']['dateTime']))
    
    Now, if you really want some proof the event was created, one of the fields that's created is a link to the calendar event. We don't use it in the code, but you can... just use e['htmlLink'].

    Regardless, that's pretty much the entire script save for the OAuth2 code that we're so familiar with from previous posts. The script is posted below in its entirety, and if you run it, depending on the date/times you use, you'll see something like this:
    $ python gcal_insert.py
    *** 'Dinner with friends' event added:
        Start: 2015-09-15T19:00:00-07:00
        End:   2015-09-15T22:00:00-07:00
    It also works with Python 3 with one slight nit/difference being the "b" prefix on from the event name due to converting from Unicode to bytes:
    *** b'Dinner with friends' event added:

    Conclusion

    There can be much more to adding a calendar event, such as events that repeat with a recurrence rule, the ability to add attachments for an event, such as a party invitation or a PDF of the show tickets. For more on what you can do when creating events, take a look at the docs for events().insert() as well as the corresponding developer guide. All of the docs for the Google Calendar API can be found here. Also be sure to check out the companion video for this code sample. That's it!

    Below is the entire script for your convenience which runs on both Python 2 and Python 3 (unmodified!):
    from __future__ import print_function
    from apiclient.discovery import build
    from httplib2 import Http
    from oauth2client import file, client, tools
    
    try:
        import argparse
        flags = argparse.ArgumentParser(parents=[tools.argparser]).parse_args()
    except ImportError:
        flags = None
    
    SCOPES = 'https://www.googleapis.com/auth/calendar'
    store = file.Storage('storage.json')
    creds = store.get()
    if not creds or creds.invalid:
        flow = client.flow_from_clientsecrets('client_secret.json', SCOPES)
        creds = tools.run_flow(flow, store, flags) \
                if flags else tools.run(flow, store)
    CAL = build('calendar', 'v3', http=creds.authorize(Http()))
    
    GMT_OFF = '-07:00'      # PDT/MST/GMT-7
    EVENT = {
        'summary': 'Dinner with friends',
        'start':  {'dateTime': '2015-09-15T19:00:00%s' % GMT_OFF},
        'end':    {'dateTime': '2015-09-15T22:00:00%s' % GMT_OFF},
        'attendees': [
            {'email': 'friend1@example.com'},
            {'email': 'friend2@example.com'},
        ],
    }
    
    e = CAL.events().insert(calendarId='primary',
            sendNotifications=True, body=EVENT).execute()
    
    print('''*** %r event added:
        Start: %s
        End:   %s''' % (e['summary'].encode('utf-8'),
            e['start']['dateTime'], e['end']['dateTime']))
    
    You can now customize this code for your own needs, for a mobile frontend, a server-side backend, or to access other Google APIs. If you want to see another example of using the Calendar API (listing the next 10 events in your calendar), check out the Python Quickstart example or its equivalent in Java (server-side, Android), iOS (Objective-C, Swift), C#/.NET, PHP, Ruby, JavaScript (client-side, Node.js), or Go. That's it... hope you find these code samples useful in helping you get started with the Calendar API!

    EXTRA CREDIT: To test your skills and challenge yourself, try creating recurring events (such as when you expect to receive your paycheck), events with attachments, or even editing existing events.