Streamlit运行时选择other选项后无法显示新增文本输入框的问题

在运行Streamlit代码时，当在topic下拉选择框中选择'other'选项，期望出现的文本输入框无法正常显示。原代码如下：

import streamlit as st
import emailing


st.session_state['topic'] = None

with st.form(key='email'):
    user_email = st.text_input('Enter your email:', key='email')
    about = st.selectbox('what do you wanna discuss', options=('job', 'projects', 'salary', 'other'),
                         key='topic')

    if about == 'other':
        other_about = st.text_input('what do you wanna discuss about')

    message = st.text_area('message', key='message')
    button = st.form_submit_button('submit')

    string = f"""
subject: NEW ONE

from {user_email}
TOPIC: {about}
{message}
"""
    if button:
        emailing.send(string)
        st.info('success')

问题原因 #

手动设置st.session_state['topic'] = None会在每次页面刷新（包括下拉框选项变更时）重置topic的状态值，干扰了Streamlit对组件状态的自动跟踪，导致if about == 'other'的判断逻辑无法正确触发，从而无法渲染对应的文本输入框。

修复方案 #

删除手动修改session_state的代码，让Streamlit自动管理下拉框的状态。同时如果需要将'other'对应的输入内容加入邮件，可将其整合到邮件文本中。

修复后的代码：

import streamlit as st
import emailing


with st.form(key='email'):
    user_email = st.text_input('Enter your email:', key='email')
    about = st.selectbox('what do you wanna discuss', options=('job', 'projects', 'salary', 'other'),
                         key='topic')

    # 初始化变量避免未定义报错
    other_about = ""
    if about == 'other':
        other_about = st.text_input('what do you wanna discuss about', key='other_topic')

    message = st.text_area('message', key='message')
    button = st.form_submit_button('submit')

    # 根据选项拼接主题内容
    topic_content = about if about != 'other' else f"{about}: {other_about}"
    string = f"""
subject: NEW ONE

from {user_email}
TOPIC: {topic_content}
{message}
"""
    if button:
        emailing.send(string)
        st.info('success')

内容的提问来源于stack exchange，提问作者grebtor