Here your task is to Create a (nice) Christmas Tree.
You don't have to check errors or incorrect input values, every thing is ok without bad tricks, only one int parameter as input and a string to return;-)...
So what to do?First three easy examples:
````
Input: 3 and Output:
  *
 ***
*****
 ###
 Input 9 and Output:
    *
   ***
  *****
   ***
  *****
 *******
  *****
 *******
*********
   ###
 Input 17 and Output:
      *
     ***
    *****
     ***
    *****
   *******
    *****
   *******
  *********
   *******
  *********
 ***********
  *********
 ***********
*************
     ###
Really nice trees, or what???! So merry Christmas;-)
````
You can see, always a root, always steps of hight 3, tree never smaller than 3 (return "") and no difference for input values like 15 or 17 (because (int) 15/3 = (int) 17/3). That's valid for every input and every tree.
I think there's nothing more to say - perhaps look at the testcases too;-)!
There are some static tests at the beginning and many random tests if you submit your solution. 
Hope you have fun:-)!
def christmas_tree(height):
    h = height - height % 3
    n, r = h // 3, ''
    sz, l1, l2, l3, k = 5 + (n - 1) * 2, '*', '***', '*****', '\r\n'
    w = (sz - len(l1)) // 2
    for i in range(n):
        r += ' ' * w + l1 + k
        r += ' ' * (w-1) + l2 + k        
        r += ' ' * (w-2) + l3 + k
        w -= 1
        l1 += '**'
        l2 += '**'
        l3 += '**'
    return r + ' ' * ((sz - 3) // 2) + '###' if height>2 else ''

def christmas_tree(height):
    h = height - height % 3
    n, r = h // 3, ''
    sz, l1, l2, l3, k = 5 + (n - 1) * 2, '*', '***', '*****', '\r\n'
    w = (sz - len(l1)) // 2
    for i in range(n):
        r += ' ' * w + l1 + k
        r += ' ' * (w-1) + l2 + k        
        r += ' ' * (w-2) + l3 + k
        w -= 1
        l1 += '**'
        l2 += '**'
        l3 += '**'
    return r + ' ' * ((sz - 3) // 2) + '###' if height>2 else ''
