This must be the result of an optimization of some sort? If you inherit from str or unicode and override the __init__, the __init__ will get called, but it's subject to the exact same signature as the base str or unicode type. Probably just need to override __new__() since str/unicodes are supposed to be immutable anyhoo. (PS, have I always had …

pdw_unicode_str_init.py

class TestStr(str):                                                                                                                                                                                                         
    def __init__(self, a, b=True):                                                                                                                                                                                          
        if b:                                                                                                                                                                                                              
            print(b'using custom init for sure')                                                                                                                                                                            
        super(TestStr, self).__init__(a)  

>>> core.TestStr('hi')
using custom init for sure
'hi'
>>> core.TestStr('hi', False)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: str() takes at most 1 argument (2 given)


class TestUnicode(unicode):                                                                                                                                                                                                
    def __init__(self, a, b=True):                                                                                                                                                                                          
        if b:                                                                                                                                                                                                              
            print('using custom init for sure')                                                                                                                                                                             
        super(TestUnicode, self).__init__(a) 


>>> core.TestUnicode('hi')
using custom init for sure
u'hi'
>>> core.TestUnicode('hi', True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unicode() argument 2 must be string, not bool
>>> core.TestUnicode('hi', 'wat')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
LookupError: unknown encoding: wat