How to validate html forms in python Flask?

Question

I have a HTML form inside a landing page and I want to send the forms data by ajax to /data rout.
The problem is validating this HTML form in backend.
I know about Flask WTF form but using this method will generate form on backend which is not my case.

from flask import Flask, request, url_for
...
@app.route("/data", methods=["GET", "POST"])
def get_data():
if request.method == "POST":
    username = request.form["username"]
    ...  

My html form:

<form method="POST" action="">
<input type="text" name="username">
<input type="submit" value="Send">
</form>  

One hard way is to validate every field using regex and write several if conditions for them. I'm wondering for easier way?

Solution

Using Webargs

from webargs import flaskparser, fields

FORM_ARGS = {
    'email': fields.Email(required=True),
    'username': fields.Str(required=True),

@app.route("/data", methods=["GET", "POST"])
def get_data():
    if request.method == "POST":
        parsed_args = flaskparser.parser.parse(FORM_ARGS, request)

But as long as you know the format of the incoming data you can still use WTFs for capturing the posted info (you dont need to render WTForms on a page for it to work), for example:

# import blurb    

class Form(FlaskForm):
    username = StringField('Username', validators=[InputRequired()])
    email = EmailField('Email', validators=[InputRequired(), Length(4, 128), Email()])

@app.route("/data", methods=["GET", "POST"])
def get_data():
    if request.method == "POST":
        form = Form()  # will register fields called 'username' and 'email'.
        if form.validate_on_submit():
            # do something