>>> 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.

It's currently implemented as a positional argument, not a keyword argument. Thus the "/" at the end of the signature in the help text. That also suggests that it was a conscious decision, not an oversight. Personally, I'd also prefer it if it was available as a keyword argument.

Is this about sum not accepting start as keyword argument? It's fixed with issue34637. The help function in the report is that start could only be a positional argument with start appearing before '/' in "sum(iterable, start=0, /)" . This has been changed in 3.8 to below signature.

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.

$ python3.7
Python 3.7.1rc2 (v3.7.1rc2:6c06ef7dc3, Oct 13 2018, 05:10:29)
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> sum(range(10), start=10)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: sum() takes no keyword arguments

# 3.8

$ ./python.exe
Python 3.8.0a3+ (heads/master:7444daada1, Mar 30 2019, 20:27:47)
[Clang 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> sum(range(10), start=10)
55

Yes, duplicate of issue34637.