string vs bytes type checking/handling

[oberstet]

Split out from #11

The way the type checking is done in the code base problematic. Take this line:

if not isinstance(var, six.string_types):
    raise MarshallingError('Required string. Received: ' + repr(var))

The problem is, on Python 2 this will always evaluate to False! For both bytes and unicode strings.

(cpy361_7) oberstet@thinkpad-t430s:~/scm/crossbario/crossbar-fabric-device-agent$ python2
Python 2.7.12 (default, Nov 19 2016, 06:48:10) 
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import six
>>> six.text_type
<type 'unicode'>
>>> six.binary_type
<type 'str'>
>>> type(b'hello') == six.binary_type
True
>>> type(b'hello') == six.text_type
False
>>> six.string_types
(<type 'basestring'>,)
>>> isinstance(u'hello', six.string_types)
True
>>> isinstance(b'hello', six.string_types)
True
>>> 
(cpy361_7) oberstet@thinkpad-t430s:~/scm/crossbario/crossbar-fabric-device-agent$ python
Python 3.6.1 (default, Apr  2 2017, 18:19:20) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import six
>>> six.text_type
<class 'str'>
>>> six.binary_type
<class 'bytes'>
>>> type(b'hello') == six.binary_type
True
>>> type(b'hello') == six.text_type
False
>>> six.string_types
(<class 'str'>,)
>>> isinstance(u'hello', six.string_types)
True
>>> isinstance(b'hello', six.string_types)
False
>>> 

The reason is, that on Python 2, both str (bytes) and unicode derive of basestring (https://pythonhosted.org/six/#six.string_types, https://docs.python.org/2/library/functions.html#basestring).

The right way to check is via six.text_type and six.binary_type. See above.

[WhyNotHugo]

Makes perfect sense. Given the comment right above the method, I'd say that six.text_type should expected here.

I think it makes sense for this to be:

    if not isinstance(var, six.text_type):
        raise MarshallingError(
            'Required string. Received: %' % type(var).__name__
        )

Note that the error message now shows the type, rather than repr as well (does this make sense too?).