Why does docopt fail with docopt.DocoptLanguageError: unmatched '['?

Question

Why does this code fail with the following exception?

"""my_program - for doing awesome stuff

Usage: my_program [--foo]

Options:
    --foo - this will do foo
"""

import docopt
args = docopt.docopt(doc=__doc__)

Exception:

Traceback (most recent call last):
  File "/tmp/post.py", line 10, in <module>
    args = docopt.docopt(doc=__doc__)
  File "/Users/rbednark/.virtualenvs/docopt-python2/lib/python3.5/site-packages/docopt.py", line 560, in docopt
    pattern = parse_pattern(formal_usage(DocoptExit.usage), options)
  File "/Users/rbednark/.virtualenvs/docopt-python2/lib/python3.5/site-packages/docopt.py", line 373, in parse_pattern
    result = parse_expr(tokens, options)
  File "/Users/rbednark/.virtualenvs/docopt-python2/lib/python3.5/site-packages/docopt.py", line 381, in parse_expr
    seq = parse_seq(tokens, options)
  File "/Users/rbednark/.virtualenvs/docopt-python2/lib/python3.5/site-packages/docopt.py", line 396, in parse_seq
    atom = parse_atom(tokens, options)
  File "/Users/rbednark/.virtualenvs/docopt-python2/lib/python3.5/site-packages/docopt.py", line 413, in parse_atom
    result = pattern(*parse_expr(tokens, options))
  File "/Users/rbednark/.virtualenvs/docopt-python2/lib/python3.5/site-packages/docopt.py", line 381, in parse_expr
    seq = parse_seq(tokens, options)
  File "/Users/rbednark/.virtualenvs/docopt-python2/lib/python3.5/site-packages/docopt.py", line 396, in parse_seq
    atom = parse_atom(tokens, options)
  File "/Users/rbednark/.virtualenvs/docopt-python2/lib/python3.5/site-packages/docopt.py", line 415, in parse_atom
    raise tokens.error("unmatched '%s'" % token)
docopt.DocoptLanguageError: unmatched '['

docopt version: 0.6.2
python versions: 2.7.10, 3.5.1

Answer 1

It fails because of only a single space after --foo on this line:

    --foo - this will do foo

Fix it by adding another space after --foo:

    --foo  - this will do foo

Per the documentation:

Use two spaces to separate options with their informal description:

--verbose More text.   # BAD, will be treated as if verbose option had
                       # an argument "More", so use 2 spaces instead
-q        Quit.        # GOOD
-o FILE   Output file. # GOOD
--stdout  Use stdout.  # GOOD, 2 spaces

Additional reference: docopt source code