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Can you provide a code snippet and steps to reproduce the problem?

Since the value in Session State and the output of the widget is the option value and not the index, I can't generate an error from this. I tried a simple selectbox with a None option included. When choosing between None, some other option, or nothing selected, the widget always had the correct value and showed placeholder text when nothing was selected.

import streamlit as st

a = st.selectbox("A", [1, 2, None], index=None, key="key")
st.session_state.key
a

Can you provide a code snippet and steps to reproduce the problem?

Since the value in Session State and the output of the widget is the option value and not the index, I can't generate an error from this. I tried a simple selectbox with a None option included. When choosing between None, some other option, or nothing selected, the widget always had the correct value and showed placeholder text when nothing was selected.

import streamlit as st

a = st.selectbox("A", [1, 2, None], index=None, key="key")
st.session_state.key
a

here is the code：

import streamlit as st

project = st.selectbox("Select the project", options=[1, 2], key="project_selection")

if project == 1:
    dataset_option = []
elif project == 2:
    dataset_option = ["dataset1", "dataset2"]

dataset = st.selectbox("Select the dataset", options=dataset_option, key="dataset_selection", index=0 if len(dataset_option) > 0 else None)

if dataset is None:
    st.error("No datasets available.")

with this kind of situation, index will never work after project from 1 to 2.

The expected rendering should be that when 'project2' is selected, the dataset selectbox should default to 'dataset1', because index=0 if len(dataset_option) > 0 else None.

However, if you run the code, you will find that with project changes, dataset is always defalut to be None.

this problem will happen in v1.40.0 and v1.41.0. but in 1.33.0, it works as expected

That makes sense. The ability to set the initial value to None was added in 1.34.0. I was able to reproduce the issue with your code. I added another option to force a clearing of the key from Session State, as well:

import streamlit as st

st.write(st.session_state)
st.button("Rerun")

project = st.selectbox("Select the project", options=[1, 2, 3], key="project_selection")

if project == 1:
    dataset_option = []
elif project == 2:
    dataset_option = ["dataset1", "dataset2"]

if project != 3:
    dataset = st.selectbox("Select the dataset", options=dataset_option, key="dataset_selection", index=0 if len(dataset_option) > 0 else None)
    if dataset is None:
        st.error("No datasets available.")

st.write(st.session_state)

1. Load the page
2. Select project 2
3. Observe the default value is not "dataset1" as expected.
4. Select project 3
5. Click rerun to confirm the dataset key is cleared from Session State.
6. Select project 2
7. Observe the default value is "dataset1" as expected.

That makes sense. The ability to set the initial value to None was added in 1.34.0. I was able to reproduce the issue with your code. I added another option to force a clearing of the key from Session State, as well:

import streamlit as st

st.write(st.session_state)
st.button("Rerun")

project = st.selectbox("Select the project", options=[1, 2, 3], key="project_selection")

if project == 1:
    dataset_option = []
elif project == 2:
    dataset_option = ["dataset1", "dataset2"]

if project != 3:
    dataset = st.selectbox("Select the dataset", options=dataset_option, key="dataset_selection", index=0 if len(dataset_option) > 0 else None)
    if dataset is None:
        st.error("No datasets available.")

st.write(st.session_state)

1. Load the page
2. Select project 2
3. Observe the default value is not "dataset1" as expected.
4. Select project 3
5. Click rerun to confirm the dataset key is cleared from Session State.
6. Select project 2
7. Observe the default value is "dataset1" as expected.

And as I posted, Are these two lines（313-314） of code necessary? What issues would arise if they were commented out? Adding an extra button to clear st.session_state is a supplementary and redundant operation. I don't think this is a perfect solution

Apologies for the confusion. I wasn't suggesting a solution; I was trying to assist the devs for when they get back from holiday break so they can jump right to marking this as confirmed and prioritizing the fix. 🙂

这很有道理。在 1.34.0 中添加了设置初始值的功能None。我能够使用您的代码重现该问题。我还添加了另一个选项来强制从会话状态中清除密钥：

import streamlit as st

st.write(st.session_state)
st.button("Rerun")

project = st.selectbox("Select the project", options=[1, 2, 3], key="project_selection")

if project == 1:
    dataset_option = []
elif project == 2:
    dataset_option = ["dataset1", "dataset2"]

if project != 3:
    dataset = st.selectbox("Select the dataset", options=dataset_option, key="dataset_selection", index=0 if len(dataset_option) > 0 else None)
    if dataset is None:
        st.error("No datasets available.")

st.write(st.session_state)

1. 加载页面
2. 选择项目 2
3. 观察默认值不是预期的“dataset1”。
4. 选择项目 3
5. 单击“重新运行”以确认数据集键已从会话状态中清除。
6. 选择项目 2
7. 观察默认值**是否**如预期的那样为“dataset1”。

That makes sense. The ability to set the initial value to None was added in 1.34.0. I was able to reproduce the issue with your code. I added another option to force a clearing of the key from Session State, as well:

import streamlit as st

st.write(st.session_state)
st.button("Rerun")

project = st.selectbox("Select the project", options=[1, 2, 3], key="project_selection")

if project == 1:
    dataset_option = []
elif project == 2:
    dataset_option = ["dataset1", "dataset2"]

if project != 3:
    dataset = st.selectbox("Select the dataset", options=dataset_option, key="dataset_selection", index=0 if len(dataset_option) > 0 else None)
    if dataset is None:
        st.error("No datasets available.")

st.write(st.session_state)

1. Load the page
2. Select project 2
3. Observe the default value is not "dataset1" as expected.
4. Select project 3
5. Click rerun to confirm the dataset key is cleared from Session State.
6. Select project 2
7. Observe the default value is "dataset1" as expected.

And as I posted, Are these two lines（313-314） of code necessary? What issues would arise if they were commented out? Adding an extra button to clear st.session_state is a supplementary and redundant operation. I don't think this is a perfect solution

That makes sense. The ability to set the initial value to None was added in 1.34.0. I was able to reproduce the issue with your code. I added another option to force a clearing of the key from Session State, as well:

import streamlit as st

st.write(st.session_state)
st.button("Rerun")

project = st.selectbox("Select the project", options=[1, 2, 3], key="project_selection")

if project == 1:
    dataset_option = []
elif project == 2:
    dataset_option = ["dataset1", "dataset2"]

if project != 3:
    dataset = st.selectbox("Select the dataset", options=dataset_option, key="dataset_selection", index=0 if len(dataset_option) > 0 else None)
    if dataset is None:
        st.error("No datasets available.")

st.write(st.session_state)

1. Load the page
2. Select project 2
3. Observe the default value is not "dataset1" as expected.
4. Select project 3
5. Click rerun to confirm the dataset key is cleared from Session State.
6. Select project 2
7. Observe the default value is "dataset1" as expected.

btw, as the way you said "Click rerun button" will not show the "dataset1" as expected, because, a "st.rerun" is needed? I think

If you want a workaround for this until it's fixed, clearing the key from Session State would work:

import streamlit as st

def clear_key():
    del st.session_state.dataset_selection

project = st.selectbox("Select the project", options=[1, 2], key="project_selection", on_change=clear_key)

if project == 1:
    dataset_option = []
elif project == 2:
    dataset_option = ["dataset1", "dataset2"]

dataset = st.selectbox("Select the dataset", options=dataset_option, key="dataset_selection", index=0 if len(dataset_option) > 0 else None)
if dataset is None:
    st.error("No datasets available.")

btw, as the way you said "Click rerun button" will not show the "dataset1" as expected, because, a "st.rerun" is needed? I think

The st.rerun isn't technically needed. The key is cleared when the script run finishes after selecting 3. You just can't see that the key was cleared because Session State is displayed during the script run. I just added that to be complete in demonstrating the different pieces that show the bug (or not).

如果您想要一个解决方法直到问题解决，那么从会话状态中清除密钥会起作用：

import streamlit as st

def clear_key():
    del st.session_state.dataset_selection

project = st.selectbox("Select the project", options=[1, 2], key="project_selection", on_change=clear_key)

if project == 1:
    dataset_option = []
elif project == 2:
    dataset_option = ["dataset1", "dataset2"]

dataset = st.selectbox("Select the dataset", options=dataset_option, key="dataset_selection", index=0 if len(dataset_option) > 0 else None)
if dataset is None:
    st.error("No datasets available.")

I get that. And will this be

If you want a workaround for this until it's fixed, clearing the key from Session State would work:

import streamlit as st

def clear_key():
    del st.session_state.dataset_selection

project = st.selectbox("Select the project", options=[1, 2], key="project_selection", on_change=clear_key)

if project == 1:
    dataset_option = []
elif project == 2:
    dataset_option = ["dataset1", "dataset2"]

dataset = st.selectbox("Select the dataset", options=dataset_option, key="dataset_selection", index=0 if len(dataset_option) > 0 else None)
if dataset is None:
    st.error("No datasets available.")

Will this be fixed in the feature? 😂

I'll defer to the devs when they get back from holiday break. 😅 My guess is: probably.

It is the case in general that if you have a widget with key="A" on one script run (with some value assigned to it), and you have a different widget with key="A" on the next rerun (such as by changing one of the defining parameters of that widget), then Streamlit should delete the key and reinitialize it with the new widget's default value. So there might be some logic on a deeper level that needs fixing. Since the value of a previous widget is bleeding into a new one, there might be an issue a few lines up with computing and registering the widget ID instead...

Hi @vivianyang11 ,

Thank you for filing this issue! I've confirmed it and triaged the issue for fixing.

Thank you @sfc-gh-dmatthews for providing a workaround.

Laura

@sfc-gh-lwilby I noticed it also happens with st.radio, too.