Python tutorial could make rules for default argument definition clearer

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-01-27.22:01:00.254>
labels = ['3.9', 'docs']
title = 'Python tutorial could make rules for default argument definition clearer'
updated_at = <Date 2021-01-27.22:01:00.254>
user = 'https://bugs.python.org/jessevsilverman'

bugs.python.org fields:

activity = <Date 2021-01-27.22:01:00.254>
actor = 'jessevsilverman'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2021-01-27.22:01:00.254>
creator = 'jessevsilverman'
dependencies = []
files = []
hgrepos = []
issue_num = 43043
keywords = []
message_count = 1.0
messages = ['385813']
nosy_count = 2.0
nosy_names = ['docs@python', 'jessevsilverman']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue43043'
versions = ['Python 3.9']