onehop.py

a=[(2,3), (1,2), (3,1), (1,3), (3,2), (2,4), (4,1)]
##answer= [(1,2), (1,3), (1,4), (2,1), (3,2), (3,4), (4,2), (4,3)]
b=[(2,3), (1,2)]
c=[(1,2),(3,4), (5,6)]
def onehop(pairs):
    res=[]
    for i in range(len(pairs)):
        for j in range(len(pairs)):
            if pairs[i][1]==pairs[j][0] and pairs[i][0]!=pairs[j][1]: 
                res.append((pairs[i][0], pairs[j][1]))
    return sorted(set(res))                            

print(onehop(a))