Django timezones examples - why don't I get any time zones?

Question

I've just made use of the example shown in the Django doco of setting the active timezone : https://docs.djangoproject.com/en/1.8/topics/i18n/timezones/#selecting-the-current-time-zone .

The thing is I the template shown there renders with nothing in the Select box . Everything else seems fine .

Here's my version of it:

    {% load tz %}
    {% get_current_timezone as TIME_ZONE %}
    <form action="{% url 'set_timezone' %}" method="POST">
        {% csrf_token %}
        <label for="timezone">Time zone:</label>
        <select name="timezone">
            {% for tz in timezones %}
            <option value="{{ tz }}"{% if tz == TIME_ZONE %} selected="selected"{% endif %}>{{ tz }}</option>                                                                                                    {% endfor %}
        </select>
        <input type="submit" value="Set" />
    </form>

It all renders fine but the Select box is empty - Can anyone explain that ?

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EDITS IN RESPONSE TO COMMENTS

My view code looks like this :

from django.shortcuts import redirect, render

def set_timezone(request):
    if request.method == 'POST':
        request.session['django_timezone'] = request.POST['timezone']
        return redirect('/')
    else:
        return render(request, 'template.html', {'timezones': pytz.common_timezones})

I can confirm that pytz is installed. pip freeze includes :

pytz==2015.4

With respect to "where did I define timezones ?" That's sort of my question. I don't understand why the code in the doco suggests 'timezones' is just there . I wondered if it was a side effect of {% load tz %} but otherwise ... yes that's the nub of my question.

---

EDITS IN RESPONSE TO COMMENTS

@Anentropic: when you say I'm missing an endfor ... that's not my code that's out the Django doco. Maybe it's missing an endfor but if it is I can't see where the endfor should be.

For the record here's the actual template code I'm using (which is a cut and paste from that Django code)

    <!-- The set TZ form -->
    {% load tz %}
    {% get_current_timezone as TIME_ZONE %}
    <form action="{% url 'set_timezone' %}" method="POST">
        {% csrf_token %}
        <label for="timezone">Time zone:</label>
        <select name="timezone">
            {% for tz in timezones %}
            <option value="{{ tz }}"{% if tz == TIME_ZONE %} selected="selected"{% endif %}>{{ tz }}</option>
            {% endfor %}
        </select>
        <input type="submit" value="Set" />
    </form>