import streamlit as st

@st.cache_data
def fetch_and_clean_data(url):
    # Fetch data from URL here, and then clean it up.
    return data

d1 = fetch_and_clean_data(DATA_URL_1)
# Actually executes the function, since this is the first time it was
# encountered.

d2 = fetch_and_clean_data(DATA_URL_1)
# Does not execute the function. Instead, returns its previously computed
# value. This means that now the data in d1 is the same as in d2.

d3 = fetch_and_clean_data(DATA_URL_2)
# This is a different URL, so the function executes.

To set the persist parameter, use this command as follows:

import streamlit as st

@st.cache_data(persist="disk")
def fetch_and_clean_data(url):
    # Fetch data from URL here, and then clean it up.
    return data

By default, all parameters to a cached function must be hashable. Any parameter whose name begins with _ will not be hashed. You can use this as an "escape hatch" for parameters that are not hashable:

import streamlit as st

@st.cache_data
def fetch_and_clean_data(_db_connection, num_rows):
    # Fetch data from _db_connection here, and then clean it up.
    return data

connection = make_database_connection()
d1 = fetch_and_clean_data(connection, num_rows=10)
# Actually executes the function, since this is the first time it was
# encountered.

another_connection = make_database_connection()
d2 = fetch_and_clean_data(another_connection, num_rows=10)
# Does not execute the function. Instead, returns its previously computed
# value - even though the _database_connection parameter was different
# in both calls.

A cached function's cache can be procedurally cleared:

import streamlit as st

@st.cache_data
def fetch_and_clean_data(_db_connection, num_rows):
    # Fetch data from _db_connection here, and then clean it up.
    return data

fetch_and_clean_data.clear()
# Clear all cached entries for this function.

To override the default hashing behavior, pass a custom hash function. You can do that by mapping a type (e.g. datetime.datetime) to a hash function (lambda dt: dt.isoformat()) like this:

import streamlit as st
import datetime

@st.cache_data(hash_funcs={datetime.datetime: lambda dt: dt.isoformat()})
def convert_to_utc(dt: datetime.datetime):
    return dt.astimezone(datetime.timezone.utc)

Alternatively, you can map the type's fully-qualified name (e.g. "datetime.datetime") to the hash function instead:

import streamlit as st
import datetime

@st.cache_data(hash_funcs={"datetime.datetime": lambda dt: dt.isoformat()})
def convert_to_utc(dt: datetime.datetime):
    return dt.astimezone(datetime.timezone.utc)