Finding the sum of numbers in jinja2 template using flask

Question

I am learning flask and tried to create a web app that accepts a student's name and marks, finds the total of all the marks and display things in a table. But the total always gets displayed as 0.

The code is given below

mark_total.py:

from flask import Flask, render_template, request

app = Flask (__name__)

@app.route('/')
def student():
        return render_template('student.html')

@app.route('/result', methods = ['POST', 'GET'])
def result():
        if request.method == 'POST':
                result = request.form
                return render_template('result.html', result = result)

if __name__ == '__main__':
        app.run(host = '0.0.0.0', debug = True)

student.html:

        <form action = "/result" method = "POST">
                <p>Name <input type = "text" name = "Name" /></p>
                <p>Physics <input type = "text" name = "Physics" /></p>
                <p>Chemistry <input type = "text" name = "chemistry" /></p>
                <p>Maths <input type ="text" name = "Mathematics" /></p>
                <p><input type = "submit" value = "submit" /></p>
        </form>

</body>
</html>

result.html:

<html>
        <head>
                <title>Results</title>
        </head>

        <body>
                <h1>{{result["Name"]}}'s results</h1>
                {% set total = 0 %}
                <table border = 1>
                {% for key, val in result.iteritems() %}
                        {% if key != 'Name' %}
                                {% set total = total|int + val|int %}
                                <tr>
                                        <td>{{ key }}</td>
                                        <td>{{ val }}</td>
                                        <td>{{ total }}</td>
                                </tr>
                        {% endif %}
                {% endfor %}
                <tr>
                        <td>Total</td>
                        <td>{{total}}</td>
                </tr>
                </table>
        </body>
</html>

The html source of the output is as follows:

<html>
 <head>
  <title>Results</title>
 </head>
 <body>
  <h1>ABC's results</h1>
  <table border = 1>
   <tr>
    <td>Mathematics</td>
    <td>100</td>
    <td>100</td>
   </tr>
   <tr>
    <td>chemistry</td>
    <td>100</td>
    <td>100</td>
   </tr>
   <tr>
    <td>Physics</td>
    <td>100</td>
    <td>100</td>
   </tr>
   <tr>
    <td>Total</td>
    <td>0</td>
   </tr>
  </table>
 </body>
</html>

Can someone please help me understand how to solve this?

Do you really have to do the calculations in the template? Why not calculate this in `mark_total.py` and pass the result to the template? — Ilya V. Schurov, Jan 22 '17 at 16:08
Yes. I tried doing the changes in the python and it worked. I wanted to try it with templates. — Karthick S, Jan 22 '17 at 16:15

score 4 · Accepted Answer · edited May 23 '17 at 12:08

The problem is that your total defined inside the loop is not the same total defined (and accessed) outside of loop. This is what local scopes do.

You can overcome this by using a hack like this (adapted from this answer).

<html>
        <head>
                <title>Results</title>
        </head>

        <body>
                <h1>{{result["Name"]}}'s results</h1>
                {% set total = [0] %}
                <table border = 1>
                {% for key, val in result.iteritems() %}
                        {% if key != 'Name' %}
                                {% set _ = total.append(total.pop() + val|int) %}
                                <tr>
                                        <td>{{ key }}</td>
                                        <td>{{ val }}</td>
                                        <td>{{ total[0] }}</td>
                                </tr>
                        {% endif %}
                {% endfor %}
                <tr>
                        <td>Total</td>
                        <td>{{ total[0] }}</td>
                </tr>
                </table>
        </body>
</html>

But I'm really not sure that it is a good idea to do such things in a template. The main idea of templates is to separate the logic (like counting the totals) and the representation, and this is a violation of this principle.

Thanks Ilya for not only answering the question but for suggesting the right approach too. — Karthick S, Jan 23 '17 at 09:09

metmirr · Answer 2 · 2020-09-03T06:44:58.167

3

When you change variable value inside a loop does not effect outside of the loop except a dict:

{% set total = {'value': 0} %}
...
    {% if total.update({"value": c|int + total.value|int }) %} {% endif %}
...
    {{total.value}}

edited Sep 03 '20 at 06:44

answered Jan 22 '17 at 17:01

metmirr

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1

Not only a dict. Actually, any mutable object will work. – Ilya V. Schurov Jan 23 '17 at 11:09

Finding the sum of numbers in jinja2 template using flask

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