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POHow to dynamically create a derived type in the Python C-API
 

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  1. POHow to dynamically create a derived type in the Python C-API
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    <p>Assume we have the type <code>Noddy</code> as defined in the <a href="http://docs.python.org/extending/newtypes.html#adding-data-and-methods-to-the-basic-example" rel="noreferrer">tutorial on writing C extension modules for Python</a>. Now we want to create a derived type, overwriting only the <code>__new__()</code> method of <code>Noddy</code>.</p> <p>Currently I use the following approach (error checking stripped for readability):</p> <pre><code>PyTypeObject *BrownNoddyType = (PyTypeObject *)PyType_Type.tp_alloc(&amp;PyType_Type, 0); BrownNoddyType-&gt;tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE; BrownNoddyType-&gt;tp_name = "noddy.BrownNoddy"; BrownNoddyType-&gt;tp_doc = "BrownNoddy objects"; BrownNoddyType-&gt;tp_base = &amp;NoddyType; BrownNoddyType-&gt;tp_new = BrownNoddy_new; PyType_Ready(BrownNoddyType); </code></pre> <p>This works, but I'm not sure if it is The Right Way To Do It. I would have expected that I have to set the <a href="http://docs.python.org/c-api/typeobj.html#Py_TPFLAGS_HEAPTYPE" rel="noreferrer"><code>Py_TPFLAGS_HEAPTYPE</code></a> flag, too, because I dynamically allocate the type object on the heap, but doing so leads to a segfault in the interpreter.</p> <p>I also thought about explicitly calling <code>type()</code> using <code>PyObject_Call()</code> or similar, but I discarded the idea. I would need to wrap the function <code>BrownNoddy_new()</code> in a Python function object and create a dictionary mapping <code>__new__</code> to this function object, which seems silly.</p> <p>What is the best way to go about this? Is my approach correct? Is there an interface function I missed?</p> <h2>Update</h2> <p>There are two threads on a related topic on the python-dev mailing list <a href="http://mail.python.org/pipermail/python-dev/2009-July/090875.html" rel="noreferrer">(1)</a> <a href="http://mail.python.org/pipermail/python-dev/2009-August/091192.html" rel="noreferrer">(2)</a>. From these threads and a few experiments I deduce that I shouldn't set <code>Py_TPFLAGS_HEAPTYPE</code> unless the type is allocated by a call to <code>type()</code>. There are different recommendations in these threads whether it is better to allocate the type manually or to call <code>type()</code>. I'd be happy with the latter if only I knew what the recommended way to wrap the C function that is supposed to go in the <code>tp_new</code> slot is. For regular methods this step would be easy -- I could just use <a href="http://docs.python.org/c-api/descriptor.html#PyDescr_NewMethod" rel="noreferrer"><code>PyDescr_NewMethod()</code></a> to get a suitable wrapper object. I don't know how to create such a wrapper object for my <code>__new__()</code> method, though -- maybe I need the undocumented function <code>PyCFunction_New()</code> to create such a wrapper object.</p>
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    1. COAs I know, that is the way to do it:( But I'm not sure. I think so because the requirement, to overwrite the __new__ method, is kind of peculiar..
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      1. POHow to dynamically create a derived type in the Python C-API
      1. USzchenah
    2. CO@CHENZhao: In my use case, the base type is wrapping a C++ class with virtual member functions. The derived types only need to overwrite `__new__()` to allocate a different C++ class. Methods do not need do be overwritten since they call the virtual member function. Note that I solved this problem meanwhile by a completely different design using template techniques. The original question still remains, though.
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      1. USSven Marnach
    3. COIf you find no answer here maybe you can ask python-dev mailing list and come back here with the answer
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      1. USXavier Combelle
 

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