TypeError for Tutorial 10.3 Example 2

[DavidGoldsmith]

BPO	38678
Nosy	@rhettinger, @terryjreedy
PRs	bpo-38678: Improve argparse example in tutorial #17207 [3.8] bpo-38678: Improve argparse example in tutorial (GH-17207) #17212 [3.7] bpo-38678: Improve argparse example in tutorial (GH-17207) #17213

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = 'https://github.com/rhettinger'
closed_at = <Date 2019-11-18.06:10:05.315>
created_at = <Date 2019-11-03.23:31:18.434>
labels = ['3.8', 'type-bug', '3.7', '3.9', 'docs']
title = 'TypeError for Tutorial 10.3 Example 2'
updated_at = <Date 2019-11-18.06:17:31.623>
user = 'https://bugs.python.org/DavidGoldsmith'

bugs.python.org fields:

activity = <Date 2019-11-18.06:17:31.623>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2019-11-18.06:10:05.315>
closer = 'rhettinger'
components = ['Documentation']
creation = <Date 2019-11-03.23:31:18.434>
creator = 'David Goldsmith'
dependencies = []
files = []
hgrepos = []
issue_num = 38678
keywords = ['patch']
message_count = 7.0
messages = ['355914', '356273', '356291', '356839', '356844', '356846', '356847']
nosy_count = 4.0
nosy_names = ['rhettinger', 'terry.reedy', 'docs@python', 'David Goldsmith']
pr_nums = ['17207', '17212', '17213']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue38678'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

[DavidGoldsmith]

When I run the second example code of Section 10.3 of The Python Tutorial:

import argparse
from getpass import getuser
parser = argparse.ArgumentParser(description='An argparse example.')
parser.add_argument('name', nargs='?', default=getuser(), help='The name of someone to greet.')
parser.add_argument('--verbose', '-v', action='count')
args = parser.parse_args()
greeting = ["Hi", "Hello", "Greetings! its very nice to meet you"][args.verbose % 3]
print(f'{greeting}, {args.name}')
if not args.verbose:
    print('Try running this again with multiple "-v" flags!')

the greeting assignment line raises:

>>> greeting = ["Hi", "Hello", "Greetings! its very nice to meet you"][args.verbose % 3]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for %: 'NoneType' and 'int'
 
Is this an artifact of me being so far behind in my Python version:

Davids-MacBook-Air:~ DLG$ python3
Python 3.4.3 |Anaconda 2.4.0 (x86_64)| (default, Oct 20 2015, 14:27:51)
[GCC 4.2.1 (Apple Inc. build 5577)] on darwin

Or is it a bug in the example code in the doc?

[terryjreedy]

Crash means 'python stopped erroneously without a traceback'.

Same exception in 3.8 and 3.9. Argparse uses None for a count of 0. I consider this a bug. If not changed, it should be documented. As argparse is, the example needs to recode None to 0. (Or, it could add '-v' to sys.argv.) With proper line wrapping, the result could be

import argparse
from getpass import getuser
parser = argparse.ArgumentParser(description='An argparse example.')
parser.add_argument('name', nargs='?', default=getuser(),  # wrap
                    help='The name of someone to greet.')
parser.add_argument('--verbose', '-v', action='count')
args = parser.parse_args()  # new
args.verbose = 0 if args.verbose is None else args.verbose
greeting = (["Hi", "Hello", "Greetings! its very nice to meet you"] #wrap
            [args.verbose % 3])
print(f'{greeting}, {args.name}')

[rhettinger]

I'll replace this example with more representative one that works.

[terryjreedy]

Raymond's patch changes the tutorial example to something more useful for beginners. It also gives the proper fix for the old example, which is to add 'default=0' after "action='count'", but puts this in the proper place, the ref manual explanation of 'count'.