Actually on this site i seeing many people who kinda like to explain their code. Thats why i asked this question, and ofc playing with code and trying to figure out how it work before write comment.

Why exactly if len(s) is odd then negative index slicing s[-mid] return substring witouht first number?
I guess this operation with negative-index slicing making algorithm much more elegant and clearful, and allow you to rid of multiple if .. else blocks. But i still cant find out how it works.

groupby() take iterable(sort of sequence) and return tuple of group key(str) and lazy iterator(itertols._grouper).
So because its lazy it will give you a value only as in needed. But you can get sum of iterators values.

for k,g in groupby(s): 
  print(type(k)) # str
  print(type(g)) # itertols._grouper
  x = sum(1 for _ in g) 
  print(f'This is a key {k} and these is a sum {x} of the each group'}

Then you need to create generator expression, which consist of

[[computing_sum_of_the_group=example above, key] for key, group in each_tuple]

Hope i did it good.

I was little confusing about this case too :)
Although, this is good practice for data validation

IMHO 6 kyu kata. Pretty intresting, as for me

I dont get it: How is it possible to palindromize 750? reversed is 057.

I think this solution not so optimal at the point of memory managment. Using copy for new str is resourcefull operation that allocate new memory for new obj.
Better one is consistant comparison, when we compare letter from end and from start of input string. .lower() method for valid compare. Only counting indenfier increase and decrease. So, if we have different letters - loop is over.

You're right and wrong at the same time. Codewars not only about algorithm training, its about finding more efficient solution too.
Anyway, its cool, when u able to implement logic/algorithm of built-in function, and this able itself give you understanding of inner logic of those functions

Damn, i wrote my solution almost 6 hours :D
Regex is powerful instrument, if u know how to use it. Anyway, i've tried

Nice kata, but i think need more details. In description nothing said about digit 1 and symbols * and # implementation. In JavaScript tests digit 1 implementation is necessary and symbols don't. Although symbols not necessary some guys implement it.

Nice solution, using replace arrow function ability

More elegant than my solution. I don't remember that for loop can set two variable in first statement, so I add variable above