argparse with subcommands difference in python 2.7 / 3.5

[stephan]

BPO	28210
Nosy	@bitdancer, @timgraham, @Mariatta
Files	test_argparse.py

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

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-09-20.08:43:54.408>
labels = ['type-bug']
title = 'argparse with subcommands difference in python 2.7 / 3.5'
updated_at = <Date 2016-09-28.21:12:42.761>
user = 'https://bugs.python.org/stephan'

bugs.python.org fields:

activity = <Date 2016-09-28.21:12:42.761>
actor = 'Mariatta'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2016-09-20.08:43:54.408>
creator = 'stephan'
dependencies = []
files = ['44751']
hgrepos = []
issue_num = 28210
keywords = []
message_count = 5.0
messages = ['277017', '277028', '277099', '277154', '277566']
nosy_count = 5.0
nosy_names = ['r.david.murray', 'paul.j3', 'Tim.Graham', 'stephan', 'Mariatta']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue28210'
versions = ['Python 2.7', 'Python 3.5']

[stephan]

In python 2.7.12 I get an error if I do not pass
arguments, while in python 3.5.2 I do not get
the error (it fails silently).
Stumbled on this during my migration of my python 2.7 code to python 3.5 for django.

Here is the console output:

D:\util\python\test>py -3 test_argparse.py
3.5.2 (v3.5.2:4def2a2901a5, Jun 25 2016, 22:18:55) [MSC v.1900 64 bit (AMD64)]
Use subparsers: True
Namespace(command=None)

D:\util\python\test>py -2 test_argparse.py
2.7.12 (v2.7.12:d33e0cf91556, Jun 27 2016, 15:19:22) [MSC v.1500 32 bit (Intel)]
Use subparsers: True
usage: test_argparse.py [-h] command ...
test_argparse.py: error: too few arguments

D:\util\python\test>

[timgraham]

The behavior change is from bpo-10424. Do you believe the new behavior is incorrect?

[stephan]

I am not an expert with argparse since I just started using it
(and it has quite a lot of features which are
not all self-explaining to me).

But if I didn't misunderstand something it should behave like the python 2.7 version:
If I do not pass any command I am told that I missed the parameter.

Or:

• are there some parameters I missed to make them behave equally?
• what is the intention to fail silently?

[timgraham]

Based on the usage output, it looks like the subcommand is required, but I'm not sure if there are cases where a subcommand could be optional. David, can you advise?

usage: test_argparse.py [-h] command ...

[paulj3]

Yes there was/is a bug that made subparsers optional.

http://bugs.python.org/issue9253