lambda *args, **kwargs: None

Question

consider:

blank_fn = lambda *args, **kwargs: None

def callback(x, y, z=''):
    print x, y, z

def perform_task(callback=blank_fn):
    print 'doing stuff'
    callback('x', 'y', z='z' )

The motivation for doing it this way is I don’t have to put in logic to check if callback has been assigned because it defaults to blank_fn which just does nothing.

This works, but is there some reason I shouldn’t do it? Is it pythonic? Is there a better way to do it? Is there a built-in for:

lambda *args, **kwargs: None

Asked By: Scruffy

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Answer 1

According to PEP8, you should “Always use a def statement instead of an assignment statement that binds a lambda expression directly to a name.” So, one thing I would change is:

def blank_fn(*args, **kwargs):
    pass

However, I think a more pythonic way to do this is:

def perform_task(callback=None):
    print 'doing stuff'
    if callback is not None:
        callback('x', 'y', z='z')

There shouldn’t be any need to call a function that does nothing. Truth value testing is cheaper than function calling.

def do_nothing(*args, **kwargs): pass
def do_something(arg, callback=do_nothing):
    a = 1 + 2
    callback('z', z='z')
def do_something_else(arg, callback=None):
    a = 1 + 2
    if callback is not None:
        callback('z', z='z')

%timeit do_something(3)
1000000 loops, best of 3: 644 ns per loop

%timeit do_something_else(3)
1000000 loops, best of 3: 292 ns per loop

Answered By: Chris Barker

Answer 2

I think the previous answer is superior, as it provides a better way to accomplish what the OP wanted to do.

However there may arguably be circumstances when you want a noop function when testing, or if you are monkey patching something.

So to answer the OP Question as asked, you can use Mock:

In [1]: from mock import Mock

In [2]: blank_fn = Mock(return_value=None)

In [3]: blank_fn()

In [4]: blank_fn("foo")

In [5]: blank_fn(bar="foo")

In [6]: blank_fn("foobar", bar="foo")

In [7]:

Answered By: DRendar

lambda *args, **kwargs: None

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