A suitable ‘do nothing’ lambda expression in python?

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This:

def do_nothing(*args):
    pass

is equivalent to:

lambda *args: None

With some minor differences in that one is a lambda and one isn’t. (For example, __name__ will be do_nothing on the function, and <lambda> on the lambda.) Don’t forget about **kwargs, if it matters to you. Functions in Python without an explicit return <x> return None. This is here:

A call always returns some value, possibly None, unless it raises an exception.

I’ve used similar functions as default values, say for example:

def long_running_code(progress_function=lambda percent_complete: None):
    # Report progress via progress_function.

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