>>> help(sum)
Help on built-in function sum in module builtins:

sum(iterable, start=0, /)
    Return the sum of a 'start' value (default: 0) plus an iterable of numbers
    
    When the iterable is empty, return the start value.
    This function is intended specifically for use with numeric values and may
    reject non-numeric types.

>>> sum([1, 2, 3], start=3)
Traceback (most recent call last):
  File "<pyshell#11>", line 1, in <module>
    sum([1, 2, 3], start=3)
TypeError: sum() takes no keyword arguments

By the way, it is very annoying that the start value is not available, because it hampers the possibility to use sum with things like vectors.