dc_fast_union.py

with open('input_pairs.txt') as f:
    ids = {}
    lines = f.readlines()
    for line in lines:
        p,q = [int(x) for x in line.split(' ')]
        if p not in ids:
            ids[p] = p
        if q not in ids:
            ids[q] = q
        i, j = p, q
        while i != ids[i]:
            i = ids[i]
        while j != ids[j]:
            j = ids[j]
        if i == j:
            continue
        ids[i] = j
        print(p,q)