In python, when inheriting from a builtin, you must be carefull as your object won't follow common rules:
>>> class A(object):
... def __init__(self, *args, **kwargs):
... print "%r" % (args,)
... print "%r" % (kwargs,)
{'prefix': 2}
(3,)
<__main__.A object at ...>
This is normal behavoir: __init__ gets called and do its job at
instantiation time of the class A.
Let's create the exact same class but inheriting from the builtin unicode:
>>> class B(unicode): ... def __init__(self, *args, **kwargs): ... print "%r" % (args,) ... print "%r" % (kwargs,) ... >>> B(3, prefix=2) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'prefix' is an invalid keyword argument for this function
Pretty traceback: __init__ of our class do not seem to be called, and because it is inheriting from unicode it seems that C level initialisation is called, bypassing rules of inheritage.
But look at this:
>>> B(3)
{}
(3,)
u'3'
Clearly, our __init__ was called ! probably after the internal init of the builtin. An idea how to check this ?
Anyway, next time, be carefull and do not inherits builtin if you don't know what you are doing.
PS: this was tested on python 2.5