def flatten(a):
 for each in a:
  if not isinstance(each,list):
   yield each
  else:
   yield from flatten(each)
if __name__ == "__main__":
 a = [[1,2],[3,[4,5]],6]
 print(list(flatten(a)))

from collections import Iterable
 
def flattern(a):
 for i in a:
  if not isinstance(i,Iterable) or isinstance(i,str):
   yield i
  else:
   yield from flattern(i)
if __name__ == "__main__":
 a = [[1,2],(3,4,tuple(5,)),["6,7,8"],[9,range(10,20,1)]]
 print(list(flattern(a)))

const flattenDeep = arr =>
 Array.isArray(arr)
 ? arr.reduce((a, b) => [...a, ...flattenDeep(b)], [])
 : [arr];