My apologies for the formatting of the last line. This is my issue corrected:

When trying to assign the metavar in add_argument() method of argparse module I'm unable to include characters after a right square bracket.

Trying to set something like this:

metavar="[host:database:collection:]field"

would raise an AssertionError, which includes:

File "/usr/lib64/python3.4/argparse.py", line 329, in _format_usage
    assert ' '.join(opt_parts) == opt_usage

However, the following is accepted:

metavar="[host:database:collection:] field"