On 30 Mar 2002, Michael Hudson wrote:

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Kevin Jacobs writes:

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Suppose I define:

class Foo(object): def __new__(cls): return 1

class Bar(object): def __new__(cls): return [1,2,3]

...

1

Python 2.2 returns: print Foo() print Bar()

...

[]

I would expect that Bar() should return [1,2,3]. Am I running into some clever undocumented feature or a bug?

Is tp_init being called on the returned list?

Correct.

I suspect that the object construction code would check the result of tp_new to see if the expected type. If not, tp_init should not be called on the instance. I suspect that this check must already be there, or else none of these cases would work at all.

It calls the tp_init of the returned type.

Anyhow, I won't know what is really happening for sure until Monday, when I can run this through gdb. However, some more data points: returning dictionaries, tuples, strings, classic object instances, and user-defined new-style classes all seem to work. Only lists seem to behave this way.

Because only lists have a tp_init that resets the object.

Guido, can you clarify what the "correct behavior" should be for the above cases? Once I know that, I will happily supply a corrective patch if one is necessary.

Correct behavior is not to return a different type unless you know what its __init__ does. We're going to document each type's __new__ and __init__, and you're welcome to help. --Guido van Rossum (home page: http://www.python.org/~guido/)