More work on bound methods.

_drafts/python-object-replacement.md

@@ -737,13 +737,48 @@ examine the internal C structure of bound methods,
 
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-[`_PyObject_HEAD_EXTRA`](https://github.com/python/cpython/blob/2.7/Include/object.h#L64)
+The four gray fields of the struct come from
+[`PyObject_HEAD`](https://github.com/python/cpython/blob/2.7/Include/object.h#L78),
+which exist in all Python objects. The first two fields are from
+[`_PyObject_HEAD_EXTRA`](https://github.com/python/cpython/blob/2.7/Include/object.h#L66),
+and only exist when the debugging macro `Py_TRACE_REFS` is defined, in order to
+support more advanced reference counting. We can see that the `im_self` field,
+which mantains the reference to our target object, is either forth or sixth in
+the struct depending on `Py_TRACE_REFS`. If we can figure out the size of the
+field and its offset from the start of the struct, then we can set its value
+directly using `ctypes.memmove()`:
+
+{% highlight python %}
+
+ctypes.memmove(id(f) + offset, ctypes.byref(ctypes.py_object(b)), field_size)
+
+{% endhighlight %}
+
+Here, `id(f)` is the memory location of our method, which refers to the start
+of the C struct from above. `offset` is the number of bytes between this memory
+location and the start of the `im_self` field. We use
+[`ctypes.byref()`](https://docs.python.org/2/library/ctypes.html#ctypes.byref)
+to create a reference to the replacement object, `b`, which will be copied over
+the existing reference to `a`. Finally, `field_size` is the number of bytes
+we're copying, equal to the size of the `im_self` field.
+
+Well, all but one of these fields are pointers, meaning they have the same
+size, equal to
+[`ctypes.sizeof(ctypes.c_void_p)`](https://docs.python.org/2/library/ctypes.html#ctypes.sizeof).
+This is (probably) 4 or 8 bytes, depending on whether you're on a 32-bit or a
+64-bit system. The other field is a `Py_ssize_t` object—_very_ likely to be
+the same size as a pointer, but we can't be sure—which is equal to
+`ctypes.sizeof(ctypes.c_ssize_t)`.
 
 [`PyCFunctionObject`](https://github.com/python/cpython/blob/2.7/Include/methodobject.h#L81)
 
+### Dictionary keys
+
+...
+
 ### Closure cells
 
-function closures
+...
 
 ### Frames