Send a non-declared variable through HTML post method

I’ve been learning python’s flask module to create websites/apis.
I was trying to make a site where I can create a user, log into that user and change the user’s password.

Here’s my code. I wanted to send a post request with the user’s username and current password and new password from /cp_user/<username> to /cp_func

# environ and getenv have been imported from os module

def users():
    with open("users", "r") as usersfile:
        userslist = usersfile.read().splitlines()
    return userslist

@app.route("/cp_func", methods=["POST"])
def cp_func():
    username=request.form["username"].strip()
    password=request.form["password"].strip()
    new_password=request.form["new_password"].strip()
    if username == "":
        return """<h1>username cannot be empty!</h1>
        <p>you have entered an empty username!</p>
        <a href="http://127.0.0.1:5000/">Return to main site</a>"""
    if password == "":
        return """<h1>current password cannot be empty!</h1>
        <p>you have entered an empty current password!</p>
        <a href="http://127.0.0.1:5000/">Return to main site</a>"""
    if new_password == "":
        return """<h1>new password cannot be empty!</h1>
        <p>you have entered an empty new password!</p>
        <a href="http://127.0.0.1:5000/">Return to main site</a>"""
    if username in users():
        if getenv(username+":PASS") == None:
            environ[username+":PASS"] = new_password
            success = True
        elif getenv(username+":PASS") == password:
            environ[username+":PASS"] = new_password
            success = True
        else:
            success = False
        if success:
            return """<form action = "http://localhost:5000/home" method = "post">
            <h1>Password changed!</h1>
            <p>Your password for account `{0}` was changed to `{1}`</p>
            <input type = "submit" value = "Goto Home">
            <a href="http://127.0.0.1:5000/">Return to main site</a>
            </form>""".format(username, new_password)
        else:
            return """<h>Access Denied</h1>
            <p>you wanted to change the password of `{0}` but failed miserably by entering the wrong password, and what you entered was `{1}`</p>
            <a href="http://127.0.0.1:5000/">Return to main site</a>""".format(username, password)
    else:
        return """<h1>user not found</h1>
        <p>you wanted to change the password of `{0}` but failed miserably by specifying a non-existing user</p>
        <a href="http://127.0.0.1:5000/">Return to main site</a>""".format(username)

@app.route("/cp_user/<username>")
def cp_user(username):
    if username == "":
        return """<h1>username cannot be empty!</h1>
        <p>you have entered an empty username!</p>
        <a href="http://127.0.0.1:5000/">Return to main site</a>"""
    if username in users():
        return """
        <form action="http://127.0.0.1:5000/cp_func" method="post">
            <table>
                <tr><td><h1>Change Password</h1></td></tr>
                <tr><td><h4>Changing username for {0}</h4></td></tr>
                <tr><td>Current Password</td>
                <td><input type="password" name="password"></td></tr>
                <tr><td>New Password</td>
                <td><input type="password" name="new_password"></td></tr>
                <tr><td><input type="submit" value="Change"></td></tr>
                <tr><td><a href="http://127.0.0.1:5000/">Go back to the main site</a></td></tr>
            </table>
        </form>
        """.format(username)
    else:
        return """<h1>user not found</h1>
        <p>you wanted to change the password of `{0}` but failed miserably by specifying a non-existing user</p>
        <a href="http://127.0.0.1:5000/">Return to main site</a>""".format(username)

here’s the HTML separately, if you needed it

        <form action="http://127.0.0.1:5000/cp_func" method="post">
            <table>
                <tr><td><h1>Change Password</h1></td></tr>
                <tr><td><h4>Changing username for {0}</h4></td></tr>
                <tr><td>Current Password</td>
                <td><input type="password" name="password"></td></tr>
                <tr><td>New Password</td>
                <td><input type="password" name="new_password"></td></tr>
                <tr><td><input type="submit" value="Change"></td></tr>
                <tr><td><a href="http://127.0.0.1:5000/">Go back to the main site</a></td></tr>
            </table>
        </form>

I’m still learning about HTML, and I’ve been searching for a solution for 5 hours straight but still couldn’t find a solution to my problem.

Thanks in advance for helping me.

>Solution :

The form needs an input field with the username. You can use a hidden input for this.

    if username in users():
        return """
        <form action="http://127.0.0.1:5000/cp_func" method="post">
            <table>
                <tr><td><h1>Change Password</h1></td></tr>
                <tr><td><h4>Changing username for {0}</h4><input type="hidden" name="username" value="{1}"></td></tr>
                <tr><td>Current Password</td>
                <td><input type="password" name="password"></td></tr>
                <tr><td>New Password</td>
                <td><input type="password" name="new_password"></td></tr>
                <tr><td><input type="submit" value="Change"></td></tr>
                <tr><td><a href="http://127.0.0.1:5000/">Go back to the main site</a></td></tr>
            </table>
        </form>
        """.format(username, username)