from typing import Counter


def mode(arr):
    """
    Calculates the mode of a list of numbers as a tuple containing the
    most frequently occurring element in the first slot, and the number
    of times it occurs in the second. If there are multiple modes,
    returns only one.

    Usage: mode([1, 1, 2, 3]) # (1, 2)
    """
    return Counter(arr).most_common(1)[0]