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>

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

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?

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 ...

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

http://bugs.python.org/issue9253