Question

I am trying to do a matplotlib bar chart on django. The data that will be used, will be queried from my database.

I came across this error when I run my code:

UnboundLocalError at /graph.png local variable ‘topic’ referenced
before assignment

The code for matplotlib part are all correct as I tried running static data before and it worked. However, when I tried quering data from database, it causes this error on views.py. I have done this query many times before on different pages and working fine. But it doesn’t work for this request. Why?

def bar_chart(request):
    #Topic Distribution
    topics = list(topic.objects.filter(subject_id=subj_id).order_by('id').values())

    fig = Figure()
    ax = fig.add_subplot(111)

    N = 5
    ind = np.arange(N)
    width = 0.35

    group_labels = [] #list of x-axis tick labels
    y = [] #list of y-values
    total_marks = 0 #total marks
    for topic in topics:
        t_questions = question.objects.filter(topic_id=topic.id)
        if (len(t_questions) != 0):
            topic_marks = 0 #each topic starts at 0 marks distribution
            for t_question in topic_questions:
                topic_marks += t_question.marks #accumulate the marks
            total_marks += topic_marks
            y.append(topic_marks)
            group_labels.append(topic.title)
    for yval in y:
        yval = yval/total_marks * 100 #convert to percentage


    ax.bar(ind, y, width, color='r')
    ax.set_ylabel('Distribution in %')
    ax.set_title('Topic Distribution')
    ax.set_xticks(ind)
    ax.set_xticklabels(group_labels)
    fig.autofmt_xdate()
    canvas=FigureCanvas(fig)
    response=HttpResponse(content_type='image/png')
    canvas.print_png(response)

    return response

Answer 1

The error is here:

topics = list(topic.objects.filter(subject_id=subj_id).order_by('id').values())

If you are following the common naming conventions for models, yours is probably called Topic, so it should be

topics = list(Topic.objects.filter(subject_id=subj_id).order_by('id').values())

Otherwise you are likely missing an import.

It’s caused by the local variable topic defined within the function and overshadowing the global one.