defaultdict.py

# defaultdict use case
# Finding a Pareto front, where (x1, x2) is a list of tuples satisfying
# e.g. (x1, x2) : |df[(df[var1 > x1]) & df[var2 > x2)]| < 0.1|df|,
# where |df| is the size of df, i.e., df.shape[0] (in Pandas)

pareto = defaultdict(lambda: np.Inf)
for x1, x2 in xs:
    pareto[x1] = min(pareto[x1], x2)