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| -rw-r--r-- | Doc/ext/newtypes.tex | 24 |
1 files changed, 24 insertions, 0 deletions
diff --git a/Doc/ext/newtypes.tex b/Doc/ext/newtypes.tex index c296adb..308e75d 100644 --- a/Doc/ext/newtypes.tex +++ b/Doc/ext/newtypes.tex @@ -160,6 +160,18 @@ to \class{noddy.Noddy}. This is so that Python knows how much memory to allocate when you call \cfunction{PyObject_New()}. +\note{If you want your type to be subclassable from Python, and your +type has the same \member{tp_basicsize} as its base type, you may +have problems with multiple inheritance. A Python subclass of your +type will have to list your type first in its \member{__bases__}, or +else it will not be able to call your type's \method{__new__} method +without getting an error. You can avoid this problem by ensuring +that your type has a larger value for \member{tp_basicsize} than +its base type does. Most of the time, this will be true anyway, +because either your base type will be \class{object}, or else you will +be adding data members to your base type, and therefore increasing its +size.} + \begin{verbatim} 0, /* tp_itemsize */ \end{verbatim} @@ -384,6 +396,18 @@ We don't fill the \member{tp_alloc} slot ourselves. Rather base class, which is \class{object} by default. Most types use the default allocation. +\note{If you are creating a co-operative \member{tp_new} (one that +calls a base type's \member{tp_new} or \method{__new__}), you +must \emph{not} try to determine what method to call using +method resolution order at runtime. Always statically determine +what type you are going to call, and call its \member{tp_new} +directly, or via \code{type->tp_base->tp_new}. If you do +not do this, Python subclasses of your type that also inherit +from other Python-defined classes may not work correctly. +(Specifically, you may not be able to create instances of +such subclasses without getting a \exception{TypeError}.)} + + We provide an initialization function: \begin{verbatim} |