Python-сообщество

lst  = [3 for x in range(0, 51)]
lst1 = [lst for x in range(0, 51)]
print(lst1)
# либо так, нагляднее:
for i in lst1:
    print(i)

[[3 for j in range(50)] for i in range(50)]

[3] * 50

import numpy as np
arr = np.zeros((50, 50), dtype=int)
arr[:] = 3
print(arr)

lst=[[3 for x in range(50)] for x in range(50)]
for i in lst:
    print(i)

def test1():
    lst = [[3 for x in range(50)] for x in range(50)]
def test2():
    lst = [[3 for x in xrange(50)] for x in xrange(50)]
def test3():
    lst = [[3] * 50 for _ in range(50)]
def test4():
    lst = [[3] * 50 for _ in xrange(50)]
if __name__ == '__main__':
    import timeit
    print(timeit.timeit("test1()", setup="from __main__ import test1", number=100000))
    print(timeit.timeit("test2()", setup="from __main__ import test2", number=100000))
    print(timeit.timeit("test3()", setup="from __main__ import test3", number=100000))
    print(timeit.timeit("test4()", setup="from __main__ import test4", number=100000))

15.5663039684
13.9283020496
2.48807406425
2.43950200081

if __name__ == '__main__':
   

...
arr = np.ones((50, 50), dtype=int)*3
...