'Generates a random string of upper and lowercase letters.
:type min_chars: int
:type max_chars: int
:return: String. Random of random length between min and max characters.'

@classmethod
def pystr(cls, min_chars=None, max_chars=20):

    if (min_chars is None):
        return u''.join((cls.random_letter() for i in range(max_chars)))
    else:
        assert (max_chars >= min_chars), u'Maximum    length    must    be    greater    than    or    equal    to    minium    length'
        return u''.join((cls.random_letter() for i in range(0, random.randint(min_chars, max_chars))))
