Streamlit multiselect doesn't perform properly

I wrote the code as follows for a Streamlit app. I read a DataFrame which is the output of a function. The code works in a way that I can select an item in a st.multiselect, however when I select an item, it runs and doesn't update the DataFrame. How should I fix this issue?

#CODE is the dataframe columns that I perform filtering.
def do_something():

    return df  

df = do_something()

def select(df):

    x= df.CODE.unique().tolist()
    codes= st.columns([0.1, 0.1])

    codes = codes.multiselect("Select Code", x, key={"key1"})

    if codes:
        df = df.query("CODE in @codes)

    return df

Seems like you just need to re-assign your DataFrame after/when calling select.

import streamlit as st
import pandas as pd

def do_something():
    df = pd.DataFrame({"CODE": ["A", "B", "C", "D", "E"],
                       "VALUE": [1, 2, 3, 4, 5]})
    return df

def select(df):
    x = df.CODE.unique().tolist()
    codes = st.multiselect("Select Code", x, key="key1")

    if codes:
        df = df.query("CODE in @codes")

    return df

df = do_something()
st.write("BEFORE:")
st.write(df)

df = select(df) # <---- add this line
st.write("AFTER:")
st.write(df)

Output :