This is just begging for a format string.
```python
tweet_action = "http://twitter.com/intent/tweet?text=I+just+registered+for+{number}+{title}+through+{account}:+{url}".format(
    number=course.number,
    title=get_course_about_section(course, 'title'),
    account=microsite.get_value('course_about_twitter_account', '@edxonline'),
    url="http://{domain}{path}".format(
        domain=site_domain,
        path=reverse('about_course', args=[course.id])
    )
)
```