Decorators with optional arguments in Python

Python decorators are one of the best features of the language, and I think that this SO answer describes them the best. I’ll take an example similar to the example from that question itself. We want to write a decorator that, when used to decorate a function that returns a string, will wrap that string with the HTML bold tags.

Concretely, given a function say_hello(),

def say_hello(user):
    return 'hello, {}'.format(user)

We should be able to do this

@makebold
def say_hello(user):
    return 'hello, {}'.format(user)

And call it to get this:

>>> say_hello('lama')
'<b>hello, lama</b>'
>>>

Doing that is simple. The makebold decorator can be written as

def makebold(func):
    def inner(*args, **kwargs):
        return '<b>{}</b>'.format(func(*args, **kwargs))
    return inner

Now what if we wanted to optionally specify a colour for the greeting? Sure, we could write another decorator colour that takes as an argument the colour. But to build up on this contrived example, say I want to pass that colour on to the makebold decorator when I want to specify a colour other than black. Concretely, we should be able to do this

@makebold
def say_hello(user):
    return 'hello, {}'.format(user)

@makebold(colour='red')
def say_bye(user):
    return 'See you, {}'.format(user)

And get output like this

>>> say_hello('Lama') 
<b>hello, Lama</b>
>>> say_bye('Lama')
<span style="color: red;"><b>See you, Lama</b></span>
>>> 

Now if we simply add an optional colour parameter to our makebold decorator, it will look like this

def makebold(colour=None):
    def decorator(func):
        colour_open = ('<span style="color: {};">'.format(colour)
                       if colour else '')
        colour_close = '</span>' if colour else ''
        def inner(*args, **kwargs):
            return ('{colour_open}<b>{message}</b>{colour_close}'
                    .format(colour_open=colour_open,
                            colour_close=colour_close,
                            message=func(*args, **kwargs)))
        return inner
return decorator

The thing to note is that we can no longer just return a function from the decorator that does the boldification. We must, instead, return a function that is the actual decorator that takes the function.

But then, the decoration syntax changes to

@makebold()
def say_hello(user):
    return 'hello, {}'.format(user)

@makebold(colour='red')
def say_bye(user):
    return 'See you, {}'.format(user)

Close enough, but definitely very ugly.

To achieve what we are after, we’ll exploit the default function arguments in Python. It turns out that the decorator we are after can be written like this

def makebold(func=None, colour=None):
    def decorator(func):
        colour_open = ('<span style="color: {};">'.format(colour)
                       if colour else '')
        colour_close = '</span>' if colour else ''
        def inner(*args, **kwargs):
            return ('{colour_open}<b>{message}</b>{colour_close}'
                    .format(colour_open=colour_open,
                            colour_close=colour_close,
                            message=func(*args, **kwargs)))
        return inner
    if func is not None:
        return decorator(func)
    return decorator

Now we can say this

@makebold
def say_hello(user):
    return 'hello, {}'.format(user)

@makebold(colour='red')
def say_bye(user):
    return 'See you, {}'.format(user)

And the calls will give us the expected output:

>>> say_hello('Lama') 
<b>hello, Lama</b>
>>> say_bye('Lama')
<span style="color: red;"><b>See you, Lama</b></span>
>>> 

Explanation

In the first decoration, we just decorate using @makebold and not @makebold(). That passes our say_hello() function as the first parameter func to the makebold decorator, as we we meant this

say_hello = makebold(say_hello)

say_hello is passed as the first positional argument, which is, according to the signature of makebold, func. Now, on line 12 in the latest definition of makebold, we check if the func argument is None, which it isn’t and hence, we pass func to the actual decorator() function and return whatever is returned - which we know, will be the function inner() defined inside decorator(). Hence, everytime this decorated say_hello() is called, it is actually a version of inner() being called.

In the second case, we decorate by specifying the colour keyword argument, and very importantly, we make an explicit call to the decorator. In short, we are asking for this:

say_bye = makebold(colour='red')(say_hello)

which is conceptually equivalent to:

real_decorator = makebold(colour='red')
say_bye = real_decorator(say_bye)

In the call to makebold(), we specify just the colour kwarg, which leaves the func kwarg None (the default value). Now, on line 12, the check for non-Noneness of func fails and hence, we return the inner decorator function(which is the one that is assigned to real_decorator above), which can in turn, take say_bye() as its parameter.

Caveats

This method relies on the default argument mechanism of Python. But code like this could break it:

@makebold(colour='blue', func='yada')
def curse(user):
    return '$%$%#@$^#%^@ {}'.format(user)

This will raise a TypeError saying that an str is not callable, which happens as our decorator tries to call its func parameter. I personally don’t think this is a problem, as if we added a check to see if func was callable in makebold(), we’d also likely raise a similar TypeError if it was not.

Additionally, this method requires the decorating code to always use kwargs, which, again, is fine with me, as decorators are most of the time, part of a library and calling them with explicit kwargs is, IMHO, better than using positional args. However, there obviously are ways to write decorators that take optional positional args, similar to this:

@makebold
def say_hello(user):
    return 'hello, {}'.format(user)

@makebold('red')
def say_bye(user):
    return 'See you, {}'.format(user)

Achieving this behaviour requires a few changes in the makebold decorator, but frankly, I wouldn’t bother.