Reduced unexpected behavior

Oct 13, 2025 by wbrum.org 16 views

Introduction

Streamlit is a powerful Python library used for creating web-based data applications. One of its key features is the ability to store and manage session-state variables, which allows developers to create interactive and dynamic applications. However, when it comes to initializing values through session-state, developers may encounter some challenges. In this article, we will explore a common issue related to value initialization through session-state and provide a solution to overcome it.

The Problem

When using Streamlit's st.session_state to store and manage session-state variables, developers may encounter issues when trying to initialize values. Specifically, when using the st.free_text_select function to create a select box, the value is not initialized correctly if the key is not present in the session-state dictionary.

Example Code

if 'foobar' not in st.session_state:
    st.session_state.foobar = "calamari"
....
....
foobar = st.free_text_select("FooTheBar", options, key="foobar")

In this example, the value of the select box is not initialized with the default value "calamari" because the key "foobar" is not present in the session-state dictionary.

The Issue

The issue here is that the st.free_text_select function does not automatically initialize the value if the key is not present in the session-state dictionary. This can lead to unexpected behavior, especially when trying to reset the value of the select box after an event or when rerunning the application.

Solution

To overcome this issue, we can use the st.session_state.setdefault method to initialize the value of the select box with a default value if the key is not present in the session-state dictionary.

Example Code

st.session_state.setdefault('foobar', "calamari")
foobar = st.free_text_select("FooTheBar", options, key="foobar")

In this example, the st.session_state.setdefault method is used to initialize the value of the select box with the default value "calamari" if the key "foobar" is not present in the session-state dictionary.

Benefits

Using the st.session_state.setdefault method to initialize values through session-state provides several benefits, including:

Improved initialization: The value of the select box is initialized correctly with the default value if the key is not present in the session-state dictionary.
Reduced unexpected behavior: The solution prevents unexpected behavior, especially when trying to reset the value of the select box after an event or when rerunning the application.
Simplified code: The solution simplifies the code by eliminating the need for explicit checks and assignments.

Conclusion

In conclusion, initializing values through session-state in Streamlit can be challenging, especially when using the st.free_text_select function. However, by using the st.session_state.setdefault method, developers can overcome this issue and ensure that values are initialized correctly. This solution provides several benefits, including improved initialization, reduced unexpected behavior, and simplified code.

Best Practices

To ensure that values are initialized correctly through session-state, follow these best practices:

Use the st.session_state.setdefault method: Use the st.session_state.setdefault method to initialize values with default values if the key is not present in the session-state dictionary.
Check for key presence: Check if the key is present in the session-state dictionary before trying to access its value.
Use explicit assignments: Use explicit assignments to initialize values with default values if the key is not present in the session-state dictionary.

Introduction

In our previous article, we explored the issue of value initialization through session-state in Streamlit and provided a solution using the st.session_state.setdefault method. In this article, we will answer some frequently asked questions related to value initialization through session-state in Streamlit.

Q: What is the difference between `st.session_state` and `st.session_state.setdefault`?

A: st.session_state is a dictionary that stores the session-state variables, while st.session_state.setdefault is a method that initializes a value with a default value if the key is not present in the session-state dictionary.

Q: Why do I need to use `st.session_state.setdefault` instead of just assigning a value to `st.session_state`?

A: Assigning a value to st.session_state will only work if the key is already present in the session-state dictionary. If the key is not present, the assignment will not take effect. Using st.session_state.setdefault ensures that the value is initialized with the default value even if the key is not present.

Q: Can I use `st.session_state.setdefault` with other types of values, such as integers or floats?

A: Yes, you can use st.session_state.setdefault with other types of values, such as integers or floats. The method will initialize the value with the default value if the key is not present in the session-state dictionary.

Q: How do I reset the value of a select box after an event?

A: To reset the value of a select box after an event, you can use the st.session_state.setdefault method to initialize the value with a default value. For example:

if st.button('Reset'):
    st.session_state.setdefault('foobar', "calamari")
    foobar = st.free_text_select("FooTheBar", options, key="foobar")

Q: Can I use `st.session_state.setdefault` with multiple keys?

A: Yes, you can use st.session_state.setdefault with multiple keys. The method will initialize the values with the default values if the keys are not present in the session-state dictionary. For example:

st.session_state.setdefault('foobar', "calamari")
st.session_state.setdefault('barbaz', "qux")
foobar = st.free_text_select("FooTheBar", options, key="foobar")
barbaz = st.free_text_select("BarBaz", options, key="barbaz")

Q: How do I handle cases where the user does not select a value from the select box?

A: To handle cases where the user does not select a value from the select box, you can use the st.session_state.setdefault method to initialize the value with a default value. For example:

foobar = st.free_text_select("FooTheBar", options, key="foobar")
if foobar not in options:
    st.session_state.setdefault('foobar', "calamari")

Conclusion

In conclusion, value initialization through session-state in Streamlit can be challenging, but using the st.session_state.setdefault method can help overcome these issues. By following the best practices outlined in this article, developers can ensure that values are initialized correctly through session-state and create robust and interactive data applications with Streamlit.

Additional Resources

For more information on value initialization through session-state in Streamlit, please refer to the following resources:

By following these resources and best practices, developers can create high-quality data applications with Streamlit.