Carrying Extra State with Callback Functions

Contributed by: David Beazley and Brian K. Jones

Difficulty level: Easy

Visit the Python Cookbook: Functions Lab to test out this Shortcut in a live practice environment.

Problem

You’re writing code that relies on the use of callback functions (e.g., event handlers, completion callbacks, etc.), but you want to have the callback function carry extra state for use inside the callback function.

Solution

This Shortcut pertains to the use of callback functions that are found in many libraries and frameworks—​especially those related to asynchronous processing. To illustrate and for the purposes of testing, define the following function, which invokes a callback:

def apply_async(func, args, *, callback):
    # Compute the result
    result = func(*args)

    # Invoke the callback with the result
    callback(result)

In reality, such code might do all sorts of advanced processing involving threads, processes, and timers, but that’s not the main focus here. Instead, we’re simply focused on the invocation of the callback. Here’s an example that shows how the preceding code gets used:

>>> def print_result(result):
...     print('Got:', result)
...
>>> def add(x, y):
...     return x + y
...
>>> apply_async(add, (2, 3), callback=print_result)
Got: 5
>>> apply_async(add, ('hello', 'world'), callback=print_result)
Got: helloworld
>>>

As you will notice, the print_result() function only accepts a single argument, which is the result. No other information is passed ...