The `/` denotes that `format` is a positional-only argument; you cannot make a call like `struct.unpack_from(format=my_format, buffer=my_buffer)`, it must be `struct.unpack_from(my_format, buffer=my_buffer)` or `struct.unpack_from(my_format, my_buffer)`.  The same syntax can be used to define your own function with positional-only arguments, see https://docs.python.org/3/tutorial/controlflow.html#positional-only-parameters or https://docs.python.org/3/reference/compound_stmts.html#function-definitions