Passing a format string as a keyword argument to string.Formatter.format() was deprecated in 3.5 (issue23671). Proposed patch finishes the deprecation period and converts a warning to an error.

Python 3.5-3.6:

>>> string.Formatter().format(format_string='foo: {foo}', foo=123)
__main__:1: DeprecationWarning: Passing 'format_string' as keyword argument is deprecated
'foo: 123'

Python 3.7:

>>> string.Formatter().format(format_string='foo: {foo}', foo=123)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/string.py", line 179, in format
    "argument: 'format_string'") from None
TypeError: format() missing 1 required positional argument: 'format_string'

New changeset d571d8cd4258 by Serhiy Storchaka in branch 'default':
Issue #29193: A format string argument for string.Formatter.format()
https://hg.python.org/cpython/rev/d571d8cd4258