Task: Get circle length, area and sphere surface, volume from radius
Code:
app.py (or similar)

from flask import Flask, render_template, request
import math

….
@app.route('/circle', methods=['GET', 'POST'])
def circle():
    circlength = None
    circarea = None
    surface = None
    volume = None
    radius = None
    if request.method == 'POST':
        try:
            radius = float(request.form.get('radius', 0))
            circlength = 2 * math.pi * radius
            circarea = math.pi * (radius ** 2)
            surface  = 4*math.pi * (radius ** 2)
            volume  = 4/3*math.pi * (radius ** 3)
        except ValueError:
            circlength = "Invalid input"
            circarea = "Please enter a valid number"

    return render_template('circle.html', circlength=circlength, circarea=circarea, surface=surface, volume=volume, radius=radius)


circle.html