_mean_dispatcher() missing 1 required positional argument: 'a'

This error occurs when the function `_mean_dispatcher()` is called without passing the required argument `a`.

For example, if the function is defined as follows:

def _mean_dispatcher(a):
    if isinstance(a, (list, tuple)):
        return mean_list(a)
    elif isinstance(a, dict):
        return mean_dict(a)
    else:
        raise TypeError("Unsupported type")

And it is called without passing `a`:

_mean_dispatcher()

Then the error message will be:

TypeError: _mean_dispatcher() missing 1 required positional argument: 'a'

To fix this error, you need to make sure to pass the required argument `a` when calling the function. For example:

_mean_dispatcher([1, 2, 3])

This will call the `mean_list()` function and return the mean of the list `[1, 2, 3]`.