callback

You can use callback clauses with the following queries:

• Select

• Objects

Callbacks are used to run arbitrary code after a query completes.

Callback handlers

A callback handler is a function or coroutine that takes query results as its only parameter.

For example, you can automatically print the result of a select query using print as a callback handler:

>>> await Band.select(Band.name).callback(print)
[{'name': 'Pythonistas'}]

Likewise for an objects query:

>>> await Band.objects().callback(print)
[<Band: 1>]

Transforming results

Callback handlers are able to modify the results of a query by returning a value. Note that in the previous examples, the queries returned None since print itself returns None.

To modify query results with a custom callback handler:

>>> def uppercase_name(band):
        return band.name.upper()

>>> await Band.objects().first().callback(uppercase_name)
'PYTHONISTAS'

Multiple callbacks

You can add as many callbacks to a query as you like. This can be done in two ways.

Passing a list of callbacks:

Band.select(Band.name).callback([handler_a, handler_b])

Chaining callback clauses:

Band.select(Band.name).callback(handler_a).callback(handler_b)