human
Here's a good function header given the following:
I wrote the below code

    def validate(key, data, errors, context):
        if (not (data[key] in list_possible_values())):
            raise Invalid('"{0}"    is    not    a    valid    parameter'.format(data[key]))
    return validate

What's a good function header?
assistant
#Here's a good function header.

def in_list(list_possible_values):