Python, Flask, MySQL - Проверьте, существует ли электронная почта в базе данных

если вы используете SQLAlchemy в вашем проекте:

@app.route("/check_email")
def check_email():
    # get email from you form data
    email = request.form.get("email")

    # check if someone already register with the email
    user = Users.query.filter_by(email=email).first()
    if not user:
        # the email doesnt exist
        pass
    else:
        # the email exists
        pass

Users.query.filter_by(email=email).first() равно SQL:

SELECT * from users where email="EMAIL_FROM_FORM_DATA"

если вы используете pymsql (или что-то в этом роде):

import pymsql

@app.route("/check_email")
def check_email():
    # get email from you form data
    email = request.form.get("email")

    conn = connect(host='localhost',port=3306,user='',password='',database='essentials')
    cs1 = conn.cursor()
    params = [email]
    # cursor return affected rows
    count = cs1.execute('select * from users where email=%s', params)  # prevent SqlInject

    if count == 0:
        # count 0 email
    else:
        # the email exists
        # and if you want to fetch the user info
        user_info = cs1.fetchall()  # the user_info should be a tuple


    # close the connection
    cs1.close()
    conn.close()