class X:
    def __init__(self, l):
        self.l = l
    def __iter__(self):
        return (x for x in self.l)
a = X([1, 2])
b = X([3, 4])
print(*chain(a, a, b, b))

class X:
    def __init__(self, l):
        self.it = (x for x in l)
    def __iter__(self):
        return self.it
a = X([1, 2])
b = X([3, 4])
print(*chain(a, a, b, b))

from collections import Iterable
items = [1, [2, 3, [4], 5], 6]
def flatten(items):
    for x in items:
        if isinstance(x, Iterable):
            yield from flatten(x)
        else:
            yield x
            
print(*flatten(items))

import heapq
a = [1, 4]
b = [2, 5]
for c in heapq.merge(a, b):
    print(c, end='')

from itertools import repeat, chain
x = [1, 2]
print(*chain(*repeat(x, 2)))

import numpy as np
a = np.array([1, 2, np.nan, np.nan])
print(np.min(a),  np.nanmin(a))

import numpy as np
a = np.array([1, 2, np.nan, np.nan])
print(np.max(a),  np.nanmax(a))

import numpy as np
x = np.fmin([1, -5, 6], [0, np.nan, -1])
print(*x)

import numpy as np
x = np.fmax([1, -5, 6], [2, np.nan, -1])
print(*x)