class LoginForm(FlaskForm):
    username = StringField('Username', validators=[DataRequired()])
    password = PasswordField('Password', validators=[DataRequired()])

@app.route('/login', methods=['POST'])
def login():
    form = forms.LoginForm()
    if form.validate_on_submit():
        try:
            user = models.User.get(models.User.username == form.username.data)
        except models.DoesNotExist:
            flash("Your username or password doesn't match")
        else:
            if check_password_hash(user.password, form.password.data):
                login_user(user)
                flash("You've been logged in!")
                return redirect(url_for('index'))
            else:
                flash("Your username or password doesn't match")

<div class="modal-body">
    <form method="post" action='{{ url_for("login") }}' name="LoginForm">
        <input type="hidden" name="csrf_token" value="{{ csrf_token() }}" />
        <div class="form-group">
            <label for="username">Username</label>
            <input type="text" class="form-control" name="username" id="username" placeholder="Username">
        </div>
        <div class="form-group">
            <label for="password">Password</label>
            <input type="password" class="form-control" name="password" placeholder="Password"></p>
        </div>
        <p><button type="submit" class="btn btn-primary">Sign in</button>
            <a href="#">Forgot Password?</a>
        </p>
    </form>
</div>