Why python documentation have written optional argument like str.endswith(suffix[,start[,end]])?

Question

Why there are square brackets
When we actually write like
a.endswith('a',1,-2)

Answer 1

It's just to show that they are optional and may be omitted.
You may call the function like
a.endswith('a')
Or
a.endswith('a', 1)
Or
a.endswith('a', 1, -2)

Depending on how you need to use the function.