def _parallel_mean(avg_a, count_a, avg_b, count_b):
    """Update avg_a with avg_b."""
    idx = count_b > 0
    avg_a[idx] = (count_a[idx] * avg_a[idx] + count_b[idx] * avg_b[idx]) / (
        count_a[idx] + count_b[idx]
    )
Update avg_a with avg_b.
pysteps/verification/detcontscores.py