argparse: does not respect required args pre-populated into namespace

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-02-27.20:33:10.214>
labels = ['3.7', 'type-bug', 'library']
title = 'argparse: does not respect required args pre-populated into namespace'
updated_at = <Date 2017-11-26.12:01:19.891>
user = 'https://bugs.python.org/mhcptg'

bugs.python.org fields:

activity = <Date 2017-11-26.12:01:19.891>
actor = 'avalentino'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-02-27.20:33:10.214>
creator = 'mhcptg'
dependencies = []
files = []
hgrepos = []
issue_num = 29670
keywords = []
message_count = 7.0
messages = ['288667', '288937', '288952', '288990', '294664', '294728', '299499']
nosy_count = 5.0
nosy_names = ['terry.reedy', 'bethard', 'paul.j3', 'mhcptg', 'avalentino']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue29670'
versions = ['Python 3.7']