Web application: Flask - html - wtforms: form with two submit buttons

One of your values is "ADD", but you test for "Add". So, the equality test fails

UPDATE:

Where are you defining add() and mul()? Perhaps instead use:

if request.form['btn'] == 'Add':
    result = form.A.data + form.B.data

elif request.form['btn'] == 'Multiply':
    result = form.A.data * form.B.data

UPDATE2:

Try this to diagnose:

if request.method == 'POST' and form.validate():
    print(request.form["btn"])
    print(form.A.data, form.B.data)
    print(request.POST)

You have 3 different forms. A and B are not part of either form you are submitting.

If I understand the issue then try to do this:

from flask import render_template, redirect, url_for, flash, request

@app.route('/comp') #sample only
def comp():

    form = InputForm(request.form)
    if request.method == 'POST' and form.validate()
        if request.form['btn'] == 'Add':
            result = add(form.A.data, form.B.data)

        elif request.form['btn'] == 'Multiply':
            result = mul(form.A.data, form.B.data)
        flash(result) # see the result in the browser
        print('result') # see the result in the terminal
        return redirect(url_for('comp'))
    else:
        result = None
    return render_template('view.html', form=form, result=result)

to implement the flash in the html simple add this somewhere

{% for msg in get_flashed_messages()%}
   <h1> {{msg}} </h1>
{% endfor %}

I always add a print statement and a flash for troubleshooting purposes in order to see where the issue is.

OK so I troubleshot your code. First thing first this is the result:

Flask Code :

from flask import Flask, request, render_template, url_for, flash, redirect
from wtforms import SubmitField, Form, FloatField, validators


app = Flask(__name__)

app.config['SECRET_KEY'] = 'dev'

def mul(A,B):
    return  A*B
def add(A,B):
    return  A+B

# Form
class InputForm(Form):
    A = FloatField(
        label='A', default=0,
        validators=[validators.InputRequired()])
    B = FloatField(
        label='B', default=0,
        validators=[validators.InputRequired()])
   
       
       
@app.route('/comp', methods=['GET', 'POST'])
@app.route('/', methods=['GET', 'POST']) #this so that both url will work
def comp():

    result = ''   #already assign a variable, else will throw error
    form = InputForm(request.form)
    if request.method == 'POST': #First we check if method is post **POINT 2**
            if form.validate(): #Then if form is validate
                print('form ok') #Test if form is ok
                if request.form['btn'] == 'Add':
                    result = add(form.A.data, form.B.data)
                    print(result) 
            
                elif request.form['btn'] == 'Multiply':
                    result = mul(form.A.data, form.B.data)
                    print(result)
                
                flash(result) # **POINT 3**
                return redirect('comp') # This will make your code run forever #**4**     
            else:
                print('form no ok ') #check if form no ok
    else:
        print('request is get') # if you put the form.validate with the method condition, the form will return False as soons as you render the template

    return render_template('view.html', form=form, result=result)


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

HTML file:

  <form method="post" action="">
      <table>
        <!-- Generates form fields -->
        <!-- POINT 1 -->
        {% for field in form %}  

            <td>{{ field.name }}</td><td>{{ field }}</td>
            <td>{{ field.label }}</td>
          </tr>
          
        {% endfor %}
        
        <tr>
          <!-- Input are inside the form tag -->
          <td><input type="submit" name="btn" value="Add"></td>
          <td><input type="submit" name="btn" value="Multiply"></td>

        </tr>
      </table>
    </form>

<!-- Print result in the browser  -->
<!-- Needs to redirect to the same page -->
<!-- return redirect('comp')    -->

  {% for msg in get_flashed_messages()%}
  <h1>   {{msg}} </h1> 
  {% endfor%}

1. So the main issue was that in the HTML the form is not together with the input field, hence it was elaborating the form separately. By adding all together then it will start to work.

2. In the python file, the if request.method == 'POST' condition run simultaneously with the form hence returning the form False even before submit it.

.3 Added a Flash method so that you can see the result

.4 Add a redirect, this will flash the result to the browser, but most importantly will refresh the form and gives you possibility to add a new one.

Note that is also important to redirect the page because imagine if you want to commit it into a database, this will make you submit the form and save the changes.