Error "'CArgObject' has no len()" when using Prolog strings

[morxa]

When using Prolog strings in a fact, an error is thrown.

import pyswip
p = pyswip.Prolog()
p.assertz('some_string_fact("abc")')
print(list(p.query('some_string_fact(S)')))

This results in an error:

$ python3 pyswip_string.py 
Traceback (most recent call last):
  File "pyswip_string.py", line 23, in <module>
    print(list(p.query('some_string_fact(S)')))
  File "/usr/lib/python3.5/site-packages/pyswip/prolog.py", line 111, in __call__
    t = getTerm(swipl_list)
  File "/usr/lib/python3.5/site-packages/pyswip/easy.py", line 404, in getTerm
    res = getList(t)
  File "/usr/lib/python3.5/site-packages/pyswip/easy.py", line 420, in getList
    result.append(getTerm(head))
  File "/usr/lib/python3.5/site-packages/pyswip/easy.py", line 406, in getTerm
    res = getFunctor(t)
  File "/usr/lib/python3.5/site-packages/pyswip/easy.py", line 429, in getFunctor
    return Functor.fromTerm(t)
  File "/usr/lib/python3.5/site-packages/pyswip/easy.py", line 252, in fromTerm
    args.append(getTerm(a0 + i))
  File "/usr/lib/python3.5/site-packages/pyswip/easy.py", line 402, in getTerm
    res = _getterm_router[p](t)
  File "/usr/lib/python3.5/site-packages/pyswip/easy.py", line 387, in getString
    if PL_get_string_chars(t, byref(s), byref(slen)):
  File "/usr/lib/python3.5/site-packages/pyswip/core.py", line 555, in check_and_call
    args[i] = list_to_bytes_list(arg)
  File "/usr/lib/python3.5/site-packages/pyswip/core.py", line 487, in list_to_bytes_list
    pList = c_char_p * len(strList)
TypeError: object of type 'CArgObject' has no len()

Expected result: No error, but the unification [{'S': "abc"}]

I'm using SWI-Prolog 7.2.3 on Fedora 25 with pyswip commit 72771d9.

[xpinguin]

Apparently, check_strings shouldn't be used to decorate PL_get_string/PL_get_string_chars because char **s is actually passed as ctypes.byref, and not as a list (IMHO, it is correct):
https://github.com/yuce/pyswip/blob/master/pyswip/easy.py#L387

The problem could be fixed by commenting the following line:
https://github.com/yuce/pyswip/blob/master/pyswip/core.py#L801

While I haven't tested it thoroughly, it seems to work for my (simple) use-cases.

[yuce]

This issue was fixed with: #31