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dek: the decorator-decorator

Project description

dek decorates your decorators to diminish defects and drudgery.

Writing a Python decorator which takes no parameters is easy.

But writing a decorator with parameters requires three nested levels of function and offers several opportunities for error - and more work if you want to decorate classes like unittest.mock.patch does.

dek is a decorator for decorators that does this deftly with a single Python function some 50 lines long.


Write a decorator print_before that prints a function’s arguments with an optional label before it executes.

Without dek all is confusion:

import functools

def print_before(label='label'):
    def deferred(func):
        def wrapped(*args, **kwargs):
            print(label, args, kwargs)
            return func(*args, **kwargs)

        return wrapped

    if callable(label):
        return deferred(label)

    return deferred

dek handles all the boilerplate:

from dek import dek

def print_before(func, label='debug'):
    print(label, func)
    return func()

You can also use use defer mode for finer control over signatures:

def print_before(func, label='debug'):
    def wrapped(foo, bar):
        print(label, foo, bar)
        return func(foo, bar)

    return wrapped


Decorators can be called in many ways:

  • @print_before

  • @print_before()

  • @print_before('debug')

  • @print_before(label='debug')

  • @print_before('debug', verbose=True)

This article talks more about decorators that take parameters.

For advanced problems, the PyPi module decorator does not do what dek does, but does pretty anything else you could conceive of in a decorator library.


dek.dek(decorator, defer=False, methods=None)

(, 92-231)

Decorate a decorator so it works with or without parameters and can decorate all the members of a class.


The function being decorated


Switch between “simple” and “defer” modes


What to do with class methods when wrapping a class

dek has two modes, simple and defer. Simple mode, the default, is less work but offers less control.

In simple mode, decorator is a single function that whose first parameter is “the call that would have been made”, represented as a functools.partial().

This means the trivial decorator, the decorator that does nothing, is trivial to write:

def trivial(func):
    return func()

Decorators with parameters aren’t much harder:

def print_before(func, label='debug'):
    print(label, func.__name__, *args)
    return func()

def do_stuff(a, b='default'):
   # do stuff

# also prints 'debug do_stuff 1'

In defer mode, decorator is a function that returns a function that does the work. This is more code but more flexible.

def trivial(func):
    def wrapper(*args, **kwargs):
        return func(*args, **kwargs)

    return wrapper

def print_before(func, label='label'):
    def wrapper(foo, bar):
        print(label, foo, bar)
        return func(foo, bar)

    return wrapper

The methods parameter describe how classes (as opposed to functions or methods) are decorated. It works in either simple or defer mode.

  • If methods is None then classes are decorated like any callable. If methods is _not_ None then classes are not decorated

  • If methods is a string then only methods whose names start with that string are decorated (which means that if methods is the empty string, that all methods are decorated).

  • If methods is a callable then only methods that return true when passed to the callable are decorated

  • If methods is True then only public, non-magic methods - methods whose names do not start with _ - are decorated

  • If methods is False then methods are not decorated (and neither is the class)

(automatically generated by doks on 2020-07-09T10:13:14.486160)

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