[Pyrex] [Cython-dev] Subclassing a non-GC type

[Robert Bradshaw]

I haven't tried it out myself, but it looks fine to me. Clearing  
TPFLAGS_HAVE_GC when there are no Python object members is a great  
idea too.

- Robert

On Jan 9, 2008, at 3:08 PM, Stefan Behnel wrote:

> Hi,
>
> I tried subclassing PyUnicodeObject, which currently makes for a  
> sure crasher
> in Cython (and Pyrex, I assume). The problem is that  
> PyUnicodeObject doesn't
> implement tp_clear() and tp_traverse(), as it doesn't care about  
> cyclic
> garbage collection - it doesn't reference any other types itself,  
> so it
> doesn't have to.
>
> However, the first thing the tp_traverse/clear() functions  
> generated by Cython
> do is call their base class equivalent - without checking if they  
> are actually
> implemented.
>
> Here is a patch that catches this and works for me. However, I'm  
> not sure if
> this is the right thing to do, as it catches this at the latest  
> possible time.
> Does anyone know if this is enough to subclass a non-GC type? As I  
> said, it
> works nicely for PyUnicodeObject. I also considered using  
> "PyType_IS_GC()"
> instead, but I find it more explicit to test if the function itself is
> implemented than the more generic type test (also, the function  
> pointer is
> needed right afterwards anyway, so the impact should be near zero).
>
> BTW, shouldn't we in turn drop these two functions (and  
> TPFLAGS_HAVE_GC) for
> extension types that do not have Python object members? If this is  
> the only
> difference, I don't see why they should be there.
>
> Stefan
>
>
> # HG changeset patch
> # User Stefan Behnel <scoder at users.berlios.de>
> # Date 1199917706 -3600
> # Node ID e097bfd9b3b2115f0cec3d53c1062287dab6a816
> # Parent  59e4b70626f25058e08467fb708dad7b21b8b8d5
> fix subclassing non-GC types like 'unicode'
>
> diff -r 59e4b70626f2 -r e097bfd9b3b2 Compiler/ModuleNode.py
> --- a/Compiler/ModuleNode.py    Wed Jan 09 23:26:01 2008 +0100
> +++ b/Compiler/ModuleNode.py    Wed Jan 09 23:28:26 2008 +0100
> @@ -723,9 +723,11 @@ class ModuleNode(Nodes.Node, Nodes.Block
>          if py_attrs:
>              self.generate_self_cast(scope, code)
>          if base_type:
> +            code.putln("if (%s->tp_traverse) {" %  
> base_type.typeptr_cname)
>              code.putln(
>                      "e = %s->tp_traverse(o, v, a); if (e) return  
> e;" %
>                          base_type.typeptr_cname)
> +            code.putln("}")
>          for entry in py_attrs:
>              var_code = "p->%s" % entry.cname
>              code.putln(
> @@ -756,9 +758,11 @@ class ModuleNode(Nodes.Node, Nodes.Block
>          if py_attrs:
>              self.generate_self_cast(scope, code)
>          if base_type:
> +            code.putln("if (%s->tp_clear) {" %  
> base_type.typeptr_cname)
>              code.putln(
>                  "%s->tp_clear(o);" %
>                      base_type.typeptr_cname)
> +            code.putln("}")
>          for entry in py_attrs:
>              name = "p->%s" % entry.cname
>              code.put_xdecref(name, entry.type)
>
>
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