TypeError: The view function did not return a valid response. The function either returned None

记一次flask项目开发中遇到的无脑问题。
TypeError: The view function did not return a valid response. The function either returned None

#错误的原因是，代码中缺少return
@app.route('/history_ocr/', methods=['GET', 'POST'])
def history_ocr():
    if request.method == 'GET':
        result = operate_database.searchresult(session.get('username'))
        render_template('history.html',
                        result=result)

修改后

@app.route('/history_ocr/', methods=['GET', 'POST'])
def history_ocr():
    if request.method == 'GET':
        result = operate_database.searchresult(session.get('username'))
        # print(result[0])
        # print(len(result))
        return render_template('history.html',
                        result=result)

就没问题了。