Why does or rather how does object.__new__ work differently in these two cases

Question

Python version: "'2.7.3 (default, Apr 10 2013, 06:20:15) \n[GCC 4.6.3]'"

I have this:

>>> class testclass1(object):
    ...     pass
    ... 

>>> class testclass2(object):
    ...     def __init__(self,param):
    ...             pass
    ... 

>>> a = object.__new__(testclass1, 56)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: object.__new__() takes no parameters

>>> b = object.__new__(testclass2, 56)

>>> b
    <__main__.testclass2 object at 0x276a5d0>

Some more fun! Compare with results of testclass1 above.

>>> class testclass3(object):
    ...     def __init__(self):
    ...             pass
    ... 

>>> c = object.__new__(testclass3, 56)

>>> c
    <__main__.testclass3 object at 0x276a790>

>>> c1 = object.__new__(testclass3)

>>> c1
    <__main__.testclass3 object at 0x276a810>

My question is how does (not why does) object__new__ behave differently in these two cases? Also notice the error is kind of misleading in the first case because in the second case object.__new__ does end up taking an argument!.

Answer 1

Both object.__new__ and object.__init__ go through a carefully constructed maze of conditions that allow excess arguments in some cases, raise an error in others, and raise a warning in a very specific one. The code that implements the checks is easy enough to follow, but the reasoning behind it would likely remain inscrutable without this elucidating comment:

You may wonder why object.__new__() only complains about arguments when object.__init__() is not overridden, and vice versa.

Consider the use cases:

1. When neither is overridden, we want to hear complaints about excess (i.e., any) arguments, since their presence could indicate there's a bug.

2. When defining an Immutable type, we are likely to override only __new__(), since __init__() is called too late to initialize an Immutable object. Since __new__() defines the signature for the type, it would be a pain to have to override __init__() just to stop it from complaining about excess arguments.

3. When defining a Mutable type, we are likely to override only __init__(). So here the converse reasoning applies: we don't want to have to override __new__() just to stop it from complaining.

4. When __init__() is overridden, and the subclass __init__() calls object.__init__(), the latter should complain about excess arguments; ditto for __new__().

Use cases 2 and 3 make it unattractive to unconditionally check for excess arguments. The best solution that addresses all four use cases is as follows: __init__() complains about excess arguments unless __new__() is overridden and __init__() is not overridden (IOW, if __init__() is overridden or __new__() is not overridden); symmetrically, __new__() complains about excess arguments unless __init__() is overridden and __new__() is not overridden (IOW, if __new__() is overridden or __init__() is not overridden).

However, for backwards compatibility, this breaks too much code. Therefore, in 2.6, we'll warn about excess arguments when both methods are overridden; for all other cases we'll use the above rules.

Answer 2

The class that you are creating has its member __init__() is called by new() to handle any creation parameters but in the fist case you have no __init__ so can not pass any parameters.

Answer 3

The __new__ static method takes a class as its first argument. The other arguments will be passed into that class's __init__ method. Since your class has no __init__ method, __new__ won't accept the other arguments.

Take a look at the documentation for more info.

As for the "how", it's implemented in C (Objects/typeobject.c), but you could perform that same check with pure Python:

def __new__(cls, *args, **kwargs):
    ...

    if not hasattr(cls, '__init__') and (args or kwargs):
        raise TypeError("object.__init__() takes no parameters")

    ...