Python syntax format: var.str() vs str(var)

Hi there,

I am new in Python.

I cannot make sense of why these formats are not just the same!

I am not arguing here.  Just trying to understand why these formats are the way they are.

Current format:
len(var)
var.upper()
str(var)
var=var[4]

Why not a standard format like for example:
len(var)
upper(var)
str(var)
middle(var,4)