Intermediate calculations

Reactive calculations

Sometimes we want to compute values based on inputs. For example, if we have a numeric input x, we could compute 2 times x with a reactive calculation:

@reactive.Calc
def double_x():
    return input.x() * 2

Then we can use the reactive calculation in an output:

@output
@render.text
def txt():
    return f"x * 2 is {double_x()}"
Shiny for Python compared to R

A reactive.Calc in Shiny for Python is equivalent to a shiny::reactive() in R. The R version is also sometimes also called a reactive expression.

How does a reactive.Calc differ from an ordinary function? The primary difference is that a reactive.Calc tries to minimize the amount of work it does, by using caching. When it runs, it caches its most recent value and simply returns that cached value until the Calc is invalidated.

What does it mean to be invalidated? Reactive objects like reactive.Calcs and reactive.Effects (and @output functions) are invalidated when their reactive inputs change. For example double_x() uses input.x(), so if the user changes the value of input.x(), then double_x() gets invalidated. And in turn, double_x() invalidates any reactive objects that use it. In our example, it invalidates the output txt.

After the output txt is invalidated, it will re-execute, and when it does, it will call double_x(). Since double_x() was invalidated, it will re-execute (and not simply return a cached value), and it in turn will read input.x().

Basic example

#| standalone: true
#| layout: vertical

# A reactive Calc is used for its return value. It intelligently caches its value, and
# only re-runs after it has been invalidated -- that is, when upstream reactive inputs
# change.

from shiny import App, reactive, render, ui

app_ui = ui.page_fluid(
    ui.input_slider("x", "Choose a number", 1, 100, 50),
    ui.output_text_verbatim("txt1"),
    ui.output_text_verbatim("txt2"),
)


def server(input, output, session):
    # Each time input.x() changes, it invalidates this reactive.Calc object. If someone
    # then calls x_times_2(), it will execute the user function and return the value.
    # The value is cached, so if another function calls x_times_2(), it will simply
    # return the cached value, without re-running the function.  When input.x() changes
    # again, it will invalidate this reactive.Calc, and the cache will be cleared.
    @reactive.Calc
    def x_times_2():
        val = input.x() * 2
        print(f"Running x_times_2(). Result is {val}.")
        return val

    @output
    @render.text
    def txt1():
        return f'x times 2 is: "{x_times_2()}"'

    @output
    @render.text
    def txt2():
        return f'x times 2 is: "{x_times_2()}"'


app = App(app_ui, server)

Reactive Effects

A reactive.Effect wraps a function that runs when its reactive inputs change. They differ from reactive.Calcs in important ways:

  • A reactive.Calc is used for its return value: it is meant to be called from another reactive.Calc or a reactive.Effect, which then uses the return value from the first reactive.Calc. A reactive.Effect, on the other hand, does not have a return value, and cannot be called like a function. Once created, a reactive.Effect continues to exist, and it automatically re-executes after it has been invalidated.
  • As the name implies, a reactive.Effect is used for its side effect. In programming lingo, a side effect is when a function modifies state other than its return value. These side effects may include: writing to a file, printing to the console, or modifying a global variable.
Shiny for Python compared to R

A reactive.Effect in Shiny for Python is equivalent to a shiny::observe() in R. The R version is also sometimes also called a reactive observer.

More about outputs

In Shiny, outputs are created with the @output decorator. Although this may not look like it’s reactive, (since it doesn’t start with reactive.), it actually creates a reactive.Effect under the hood. The reactive.Effect has the side effect of putting a message into the outbound message queue.

In the example below, the user-provided function returns a string. That function is wrapped with @render.text, and then with @output. The @output decorator creates a reactive.Effect that puts the resulting string into the outbound message queue; each time the reactive inputs change, it re-executes and puts the new string in the message queue.

@output
@render.text
def txt():
    return f"The value of x is {input.x()}"