string error

[buran]

true division always returns float, so length / 2 returns float which is not allowed as index. use floor division length // 2

also don't use str as variable name . it's a type and you overshadow it, i.e. you will not be able ti use it if needed

my_str = 'foo'
print(isinstance(my_str, str))
str = 'bar'
print(isinstance(my_str, str))

Output:
True   <-- this is first print
Traceback (most recent call last):
  File "************", line 4, in <module>
    print(isinstance(my_str, str))
TypeError: isinstance() arg 2 must be a type or tuple of types