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Important

This is an experimental feature. Experimental features and their APIs may change or be removed at any time. To learn more, click here.

Function decorator to store singleton objects.

Each singleton object is shared across all users connected to the app. Singleton objects must be thread-safe, because they can be accessed from multiple threads concurrently.

(If thread-safety is an issue, consider using st.session_state to store per-session singleton objects instead.)

You can clear a memoized function's cache with f.clear().

Function signature

st.experimental_singleton(func=None, *, show_spinner=True, suppress_st_warning=False, max_entries=None, ttl=None, validate=None, experimental_allow_widgets=False)

Parameters

func (callable)

The function that creates the singleton. Streamlit hashes the function's source code.

show_spinner (boolean or string)

Enable the spinner. Default is True to show a spinner when there is a "cache miss" and the singleton is being created. If string, value of show_spinner param will be used for spinner text.

suppress_st_warning (boolean)

Suppress warnings about calling Streamlit commands from within the singleton function.

max_entries (int or None)

The maximum number of entries to keep in the cache, or None for an unbounded cache. (When a new entry is added to a full cache, the oldest cached entry will be removed.) The default is None.

ttl (float or timedelta or None)

The maximum number of seconds to keep an entry in the cache, or None if cache entries should not expire. The default is None.

validate (callable or None)

An optional validation function for cached data. validate is called each time the cached value is accessed. It receives the cached value as its only param; and it returns a bool result. If validate returns False, the current cached value is discarded, and the decorated function is called to compute a new value.

experimental_allow_widgets (boolean)

Allow widgets to be used in the singleton function. Defaults to False. Support for widgets in cached functions is currently experimental. Setting this parameter to True may lead to excessive memory use since the widget value is treated as an additional input parameter to the cache. We may remove support for this option at any time without notice.

Example

import streamlit as st

@st.experimental_singleton
def get_database_session(url):
    # Create a database session object that points to the URL.
    return session

s1 = get_database_session(SESSION_URL_1)
# Actually executes the function, since this is the first time it was
# encountered.

s2 = get_database_session(SESSION_URL_1)
# Does not execute the function. Instead, returns its previously computed
# value. This means that now the connection object in s1 is the same as in s2.

s3 = get_database_session(SESSION_URL_2)
# This is a different URL, so the function executes.

By default, all parameters to a singleton 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.experimental_singleton
def get_database_session(_sessionmaker, url):
    # Create a database connection object that points to the URL.
    return connection

s1 = get_database_session(create_sessionmaker(), DATA_URL_1)
# Actually executes the function, since this is the first time it was
# encountered.

s2 = get_database_session(create_sessionmaker(), DATA_URL_1)
# Does not execute the function. Instead, returns its previously computed
# value - even though the _sessionmaker parameter was different
# in both calls.

A singleton function's cache can be procedurally cleared:

import streamlit as st

@st.experimental_singleton
def get_database_session(_sessionmaker, url):
    # Create a database connection object that points to the URL.
    return connection

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

The @st.experimental_singleton decorator is used to cache the output of a function, so that it only needs to be executed once. This can improve performance in certain situations, such as when a function takes a long time to execute or makes a network request.

However, in some cases, the cached output may become invalid over time, such as when a database connection times out. To handle this, the @st.experimental_singleton decorator supports an optional validate parameter, which accepts a validation function that is called each time the cached output is accessed. If the validation function returns False, the cached output is discarded and the decorated function is executed again.

Best Practices

  • Use the validate parameter when the cached output may become invalid over time, such as when a database connection or an API key expires.
  • Use the validate parameter judiciously, as it will add an additional overhead of calling the validation function each time the cached output is accessed.
  • Make sure that the validation function is as fast as possible, as it will be called each time the cached output is accessed.
  • Consider to validate cached data periodically, instead of each time it is accessed, to mitigate the performance impact.
  • Handle errors that may occur during validation and provide a fallback mechanism if the validation fails.

Functions decorated with @st.experimental_singleton can contain static st elements. When a cache-decorated function is executed, we record the element and block messages produced, so the elements will appear in the app even when execution of the function is skipped because the result was cached.

In the example below, the @st.experimental_singleton decorator is used to cache the execution of the get_model function, that returns a 🤗 Hugging Face Transformers model. Notice the cached function also contains a st.bar_chart command, which will be replayed when the function is skipped because the result was cached.

import numpy as np
import pandas as pd
import streamlit as st
from transformers import AutoModel

@st.experimental_singleton
def get_model(model_type):
    # Contains a static element st.bar_chart
    st.bar_chart(
        np.random.rand(10, 1)
    )  # This will be recorded and displayed even when the function is skipped

    # Create a model of the specified type
    return AutoModel.from_pretrained(model_type)

bert_model = get_model("distilbert-base-uncased")
st.help(bert_model) # Display the model's docstring

Supported static st elements in cache-decorated functions include:

  • st.alert
  • st.altair_chart
  • st.area_chart
  • st.audio
  • st.bar_chart
  • st.ballons
  • st.bokeh_chart
  • st.caption
  • st.code
  • st.components.v1.html
  • st.components.v1.iframe
  • st.container
  • st.dataframe
  • st.echo
  • st.empty
  • st.error
  • st.exception
  • st.expander
  • st.experimental_get_query_params
  • st.experimental_set_query_params
  • st.experimental_show
  • st.form
  • st.form_submit_button
  • st.graphviz_chart
  • st.help
  • st.image
  • st.info
  • st.json
  • st.latex
  • st.line_chart
  • st.markdown
  • st.metric
  • st.plotly_chart
  • st.progress
  • st.pydeck_chart
  • st.snow
  • st.spinner
  • st.success
  • st.table
  • st.text
  • st.vega_lite_chart
  • st.video
  • st.warning

In addition to static elements, functions decorated with @st.experimental_singleton can also contain input widgets! Replaying input widgets is disabled by default. To enable it, you can set the experimental_allow_widgets parameter for @st.experimental_singleton to True. The example below enables widget replaying, and shows the use of a checkbox widget within a cache-decorated function.

import streamlit as st

# Enable widget replay
@st.experimental_singleton(experimental_allow_widgets=True)
def func():
    # Contains an input widget
    st.checkbox("Works!")

func()

If the cache decorated function contains input widgets, but experimental_allow_widgets is set to False or unset, Streamlit will throw a CachedStFunctionWarning, like the one below:

import streamlit as st

# Widget replay is disabled by default
@st.experimental_singleton
def func():
    # Streamlit will throw a CachedStFunctionWarning
    st.checkbox("Doesn't work")

func()

Let's demystify how widget replay in cache-decorated functions works and gain a conceptual understanding. Widget values are treated as additional inputs to the function, and are used to determine whether the function should be executed or not. Consider the following example:

import streamlit as st

@st.experimental_singleton(experimental_allow_widgets=True)
def plus_one(x):
    y = x + 1
    if st.checkbox("Nuke the value 💥"):
        st.write("Value was nuked, returning 0")
        y = 0
    return y

st.write(plus_one(2))

The plus_one function takes an integer x as input, and returns x + 1. The function also contains a checkbox widget, which is used to "nuke" the value of x. i.e. the return value of plus_one depends on the state of the checkbox: if it is checked, the function returns 0, otherwise it returns 3.

In order to know which value the cache should return (in case of a cache hit), Streamlit treats the checkbox state (checked / unchecked) as an additional input to the function plus_one (just like x). If the user checks the checkbox (thereby changing its state), we look up the cache for the same value of x (2) and the same checkbox state (checked). If the cache contains a value for this combination of inputs, we return it. Otherwise, we execute the function and store the result in the cache.

Let's now understand how enabling and disabling widget replay changes the behavior of the function.

Widget replay disabled

  • Widgets in cached functions throw a CachedStFunctionWarning and are ignored.
  • Other static elements in cached functions replay as expected.

Widget replay enabled

  • Widgets in cached functions don't lead to a warning, and are replayed as expected.
  • Interacting with a widget in a cached function will cause the function to be executed again, and the cache to be updated.
  • Widgets in cached functions retain their state across reruns.
  • Each unique combination of widget values is treated as a separate input to the function, and is used to determine whether the function should be executed or not. i.e. Each unique combination of widget values has its own cache entry; the cached function runs the first time and the saved value is used afterwards.
  • Calling a cached function multiple times in one script run with the same arguments triggers a DuplicateWidgetID error.
  • If the arguments to a cached function change, widgets from that function that render again retain their state.
  • Changing the source code of a cached function invalidates the cache.
  • Both st.experimental_singleton and st.experimental_memo support widget replay.
  • Fundamentally, the behavior of a function with (supported) widgets in it doesn't change when it is decorated with @st.experimental_singleton or @st.experimental_memo. The only difference is that the function is only executed when we detect a cache "miss".

All input widgets are supported in cache-decorated functions. The following is an exhaustive list of supported widgets:

  • st.button
  • st.camera_input
  • st.checkbox
  • st.color_picker
  • st.date_input
  • st.download_button
  • st.file_uploader
  • st.multiselect
  • st.number_input
  • st.radio
  • st.selectbox
  • st.select_slider
  • st.slider
  • st.text_area
  • st.text_input
  • st.time_input

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