forms.py

class LoginForm(FlaskForm):
    username = StringField('Username')
    email = StringField('Email')
    password = PasswordField('Password', validators=[DataRequired()])
    remember_me = BooleanField('Remember Me')
    submit = SubmitField('Sign In')

    def validate_submit(username, email):
		if (username.data == '') and (email.data == ''):
			raise ValidationError('You have to enter username or email!')

routes.py

#Login
@app.route('/login', methods=['GET', 'POST'])
def login():
    if current_user.is_authenticated:
        return redirect(url_for('index'))
    form = LoginForm()
    if form.validate_on_submit():
        user = User.query.filter_by(username=form.username.data.lower()).first()
        if user is None:
			user = User.query.filter_by(email=form.email.data.lower()).first()
			if user is None or not user.check_password(form.password.data):
				flash('Invalid username or password')
				return redirect(url_for('login'))
        login_user(user, remember=form.remember_me.data)
        next_page = request.args.get('next')
        if not next_page or url_parse(next_page).netloc != '':
			next_page = url_for('index')
        return redirect(next_page)
    return render_template('login.html', title='Sign In', form=form)