Using list comprehension It’s very easy to convert a two dimensional list into a flat (single dimensional) list.

flat_list = [num for childlist in lists for num in childlist]

Limitations:

• This is the optimal approach if your parent list has components that can be iterated, such as lists or strings, but it won’t work for non-literals like integers and floats.
• Another situation where this technique cannot be applied is a list with more than two dimensions.

Solution:

L = [
[1,2,3],
[
 	[4,5,6],
 	[7,8,9],
 	[
 		[10,11,12],
 	 		[13,14,15]
 	]
],
16,17,18
]


def flat_my_list(L: list):
result = []
for element in in L:
      try:
      	iter(element)
           	result.extend(flat_my_list(element))
 	except:
 		result.append(element)
return result


flat_my_list(L)

All of those criteria will be fulfilled by the function flat_my_list.