read_shx_header is a member function. When called the first parameter passed to a member function is self. So in that code shift retrieves self then calls the member function read_shx_shp_header passing 'shx' and the remaining parameters from the original call to read_shx_header. In other words, the code is inserting a parameter ('shx') into the front of the parameter list and chaining to another member function to get the actual work done.