Add PyCFunctionObjects to bound methods section.

_drafts/python-object-replacement.md

@@ -44,7 +44,7 @@ a = [1, 2, 3, 4]
 ...we are creating a list object with four integers, and binding it to the name
 `a`. In graph form:<sup><a id="ref2" href="#fn2">[2]</a></sup>
 
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+<svg width="223pt" height="44pt" viewBox="0.00 0.00 223.01 44.00" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"><g id="graph0" class="graph" transform="scale(1 1) rotate(0) translate(4 40)"><polygon fill="white" stroke="none" points="-4,4 -4,-40 219.012,-40 219.012,4 -4,4"/><g id="node1" class="node"><title>L</title><polygon fill="none" stroke="black" stroke-width="0.5" points="215.018,-36 126.994,-36 126.994,-0 215.018,-0 215.018,-36"/><text text-anchor="middle" x="171.006" y="-15" font-family="Courier,monospace" font-size="10.00">[1, 2, 3, 4]</text></g><g id="node2" class="node"><title>a</title><ellipse fill="none" stroke="black" stroke-width="0.5" cx="27" cy="-18" rx="27" ry="18"/><text text-anchor="middle" x="27" y="-13.8" font-family="Courier,monospace" font-size="14.00">a</text></g><g id="edge1" class="edge"><title>a&#45;&gt;L</title><path fill="none" stroke="black" stroke-width="0.5" d="M54.0461,-18C72.2389,-18 97.1211,-18 119.173,-18"/><polygon fill="black" stroke="black" stroke-width="0.5" points="119.339,-20.6251 126.839,-18 119.339,-15.3751 119.339,-20.6251"/></g></g></svg>
 
 In each of the following examples, we are creating new _references_ to the
 list object, but we are never duplicating it. Each reference points to the same
@@ -74,7 +74,7 @@ d = wrapper(a)
 
 {% endhighlight %}
 
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 Note that these references are all equal. `a` is no more valid a name for the
 list than `b`, `c.data`, or `L` (from the perspective of `d`, which is exposed
@@ -733,9 +733,9 @@ TypeError: readonly attribute
 Python clearly doesn't want us to re-bind bound methods, and a reasonable
 person would give up here, but we still have a few tricks up our sleeve. Let's
 examine the internal C structure of bound methods,
-[`PyMethodObject`](https://github.com/python/cpython/blob/2.7/Include/classobject.h#L31).
+[`PyMethodObject`](https://github.com/python/cpython/blob/2.7/Include/classobject.h#L31):
 
-<svg width="559pt" height="200pt" viewBox="0.00 18.00 559.03 200.00" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"><g id="graph0" class="graph" transform="scale(1 1) rotate(0) translate(4 226)"><title>%3</title><polygon fill="white" stroke="none" points="-4,4 -4,-226 555.032,-226 555.032,4 -4,4"/><g id="clust2" class="cluster"><title>cluster</title><polygon fill="none" stroke="black" stroke-width="0" points="8,-8 8,-214 272,-214 272,-8 8,-8"/><text text-anchor="middle" x="140" y="-14.8" font-family="Courier,monospace" font-size="14.00">PyMethodObject</text></g><g id="node1" class="node"><title>obj</title><polygon fill="none" stroke="black" stroke-width="0.5" points="551.048,-110 336.984,-110 336.984,-74 551.048,-74 551.048,-110"/><text text-anchor="middle" x="444.016" y="-89" font-family="Courier,monospace" font-size="10.00">&lt;__main__.A object at 0xdeadbeef&gt;</text></g><g id="node2" class="node"><title>struct</title><polygon fill="#eeeeee" stroke="none" points="24,-182 24,-202 256,-202 256,-182 24,-182"/><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-182 24,-202 256,-202 256,-182 24,-182"/><text text-anchor="start" x="27" y="-188.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">struct _object* </text><text text-anchor="start" x="161.422" y="-188.8" font-family="Courier,monospace" font-style="oblique" font-size="14.00" fill="#666666">_ob_next</text><polygon fill="#eeeeee" stroke="none" points="24,-162 24,-182 256,-182 256,-162 24,-162"/><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-162 24,-182 256,-182 256,-162 24,-162"/><text text-anchor="start" x="27" y="-168.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">struct _object* </text><text text-anchor="start" x="161.422" y="-168.8" font-family="Courier,monospace" font-style="oblique" font-size="14.00" fill="#666666">_ob_prev</text><polygon fill="#eeeeee" stroke="none" points="24,-142 24,-162 256,-162 256,-142 24,-142"/><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-142 24,-162 256,-162 256,-142 24,-142"/><text text-anchor="start" x="27" y="-148.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">Py_ssize_t </text><text text-anchor="start" x="119.415" y="-148.8" font-family="Courier,monospace" font-size="14.00">ob_refcnt</text><polygon fill="#eeeeee" stroke="none" points="24,-122 24,-142 256,-142 256,-122 24,-122"/><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-122 24,-142 256,-142 256,-122 24,-122"/><text text-anchor="start" x="26.5815" y="-128.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">struct _typeobject* </text><text text-anchor="start" x="194.609" y="-128.8" font-family="Courier,monospace" font-size="14.00">ob_type</text><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-102 24,-122 256,-122 256,-102 24,-102"/><text text-anchor="start" x="27" y="-108.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">PyObject* </text><text text-anchor="start" x="111.014" y="-108.8" font-family="Courier,monospace" font-size="14.00">im_func</text><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-82 24,-102 256,-102 256,-82 24,-82"/><text text-anchor="start" x="27" y="-88.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">PyObject* </text><text text-anchor="start" x="111.014" y="-88.8" font-family="Courier,monospace" font-size="14.00">im_self</text><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-62 24,-82 256,-82 256,-62 24,-62"/><text text-anchor="start" x="27" y="-68.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">PyObject* </text><text text-anchor="start" x="111.014" y="-68.8" font-family="Courier,monospace" font-size="14.00">im_class</text><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-42 24,-62 256,-62 256,-42 24,-42"/><text text-anchor="start" x="27" y="-48.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">PyObject* </text><text text-anchor="start" x="111.014" y="-48.8" font-family="Courier,monospace" font-size="14.00">im_weakreflist</text></g><g id="edge1" class="edge"><title>struct:f&#45;&gt;obj</title><path fill="none" stroke="black" stroke-width="0.5" d="M257,-92C280.313,-92 305.269,-92 329.087,-92"/><polygon fill="black" stroke="black" stroke-width="0.5" points="329.277,-94.6251 336.777,-92 329.277,-89.3751 329.277,-94.6251"/></g></g></svg>
+<svg width="559pt" height="200pt" viewBox="0.00 18.00 559.03 200.00" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"><g id="graph0" class="graph" transform="scale(1 1) rotate(0) translate(4 226)"><polygon fill="white" stroke="none" points="-4,4 -4,-226 555.032,-226 555.032,4 -4,4"/><g id="clust2" class="cluster"><title>cluster</title><polygon fill="none" stroke="black" stroke-width="0" points="8,-8 8,-214 272,-214 272,-8 8,-8"/><text text-anchor="middle" x="140" y="-14.8" font-family="Courier,monospace" font-size="14.00">PyMethodObject</text></g><g id="node1" class="node"><title>obj</title><polygon fill="none" stroke="black" stroke-width="0.5" points="551.048,-110 336.984,-110 336.984,-74 551.048,-74 551.048,-110"/><text text-anchor="middle" x="444.016" y="-89" font-family="Courier,monospace" font-size="10.00">&lt;__main__.A object at 0xdeadbeef&gt;</text></g><g id="node2" class="node"><title>struct</title><polygon fill="#eeeeee" stroke="none" points="24,-182 24,-202 256,-202 256,-182 24,-182"/><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-182 24,-202 256,-202 256,-182 24,-182"/><text text-anchor="start" x="27" y="-188.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">struct _object* </text><text text-anchor="start" x="161.422" y="-188.8" font-family="Courier,monospace" font-style="oblique" font-size="14.00" fill="#666666">_ob_next</text><polygon fill="#eeeeee" stroke="none" points="24,-162 24,-182 256,-182 256,-162 24,-162"/><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-162 24,-182 256,-182 256,-162 24,-162"/><text text-anchor="start" x="27" y="-168.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">struct _object* </text><text text-anchor="start" x="161.422" y="-168.8" font-family="Courier,monospace" font-style="oblique" font-size="14.00" fill="#666666">_ob_prev</text><polygon fill="#eeeeee" stroke="none" points="24,-142 24,-162 256,-162 256,-142 24,-142"/><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-142 24,-162 256,-162 256,-142 24,-142"/><text text-anchor="start" x="27" y="-148.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">Py_ssize_t </text><text text-anchor="start" x="119.415" y="-148.8" font-family="Courier,monospace" font-size="14.00">ob_refcnt</text><polygon fill="#eeeeee" stroke="none" points="24,-122 24,-142 256,-142 256,-122 24,-122"/><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-122 24,-142 256,-142 256,-122 24,-122"/><text text-anchor="start" x="26.5815" y="-128.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">struct _typeobject* </text><text text-anchor="start" x="194.609" y="-128.8" font-family="Courier,monospace" font-size="14.00">ob_type</text><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-102 24,-122 256,-122 256,-102 24,-102"/><text text-anchor="start" x="27" y="-108.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">PyObject* </text><text text-anchor="start" x="111.014" y="-108.8" font-family="Courier,monospace" font-size="14.00">im_func</text><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-82 24,-102 256,-102 256,-82 24,-82"/><text text-anchor="start" x="27" y="-88.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">PyObject* </text><text text-anchor="start" x="111.014" y="-88.8" font-family="Courier,monospace" font-size="14.00">im_self</text><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-62 24,-82 256,-82 256,-62 24,-62"/><text text-anchor="start" x="27" y="-68.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">PyObject* </text><text text-anchor="start" x="111.014" y="-68.8" font-family="Courier,monospace" font-size="14.00">im_class</text><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-42 24,-62 256,-62 256,-42 24,-42"/><text text-anchor="start" x="27" y="-48.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">PyObject* </text><text text-anchor="start" x="111.014" y="-48.8" font-family="Courier,monospace" font-size="14.00">im_weakreflist</text></g><g id="edge1" class="edge"><title>struct:f&#45;&gt;obj</title><path fill="none" stroke="black" stroke-width="0.5" d="M257,-92C280.313,-92 305.269,-92 329.087,-92"/><polygon fill="black" stroke="black" stroke-width="0.5" points="329.277,-94.6251 336.777,-92 329.277,-89.3751 329.277,-94.6251"/></g></g></svg>
 
 The four gray fields of the struct come from
 [`PyObject_HEAD`](https://github.com/python/cpython/blob/2.7/Include/object.h#L78),
@@ -790,6 +790,8 @@ Therefore, we can make our replacement like so:
 >>> offset = ptrs_in_struct * field_size + ctypes.sizeof(ctypes.c_ssize_t)
 >>> ctypes.memmove(id(f) + offset, ctypes.byref(ctypes.py_object(b)), field_size)
 4470258440
+>>> f.__self__ is b
+True
 >>> f()
 <__main__.B object at 0x10a8af290>
 
@@ -797,7 +799,40 @@ Therefore, we can make our replacement like so:
 
 Excellent—it worked!
 
-[`PyCFunctionObject`](https://github.com/python/cpython/blob/2.7/Include/methodobject.h#L81)
+There's another kind of bound method, which is the built-in variety as opposed
+to the user-defined variety we saw above. An example is `a.__sizeof__()`:
+
+{% highlight pycon %}
+
+>>> a, b = A(), B()
+>>> f = a.__sizeof__
+>>> f
+<built-in method __sizeof__ of A object at 0x10ab44b50>
+>>> replace(a, b)
+>>> f.__self__
+<__main__.A object at 0x10ab44b50>
+
+{% endhighlight %}
+
+This is stored internally as a
+[`PyCFunctionObject`](https://github.com/python/cpython/blob/2.7/Include/methodobject.h#L81).
+Let's take a look at its layout:
+
+<svg width="559pt" height="180pt" viewBox="0.00 18.00 559.03 180.00" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"><g id="graph0" class="graph" transform="scale(1 1) rotate(0) translate(4 206)"><polygon fill="white" stroke="none" points="-4,4 -4,-206 555.032,-206 555.032,4 -4,4"/><g id="clust2" class="cluster"><title>cluster</title><polygon fill="none" stroke="black" stroke-width="0" points="8,-8 8,-194 272,-194 272,-8 8,-8"/><text text-anchor="middle" x="140" y="-14.8" font-family="Courier,monospace" font-size="14.00">PyCFunctionObject</text></g><g id="node1" class="node"><title>obj</title><polygon fill="none" stroke="black" stroke-width="0.5" points="551.048,-90 336.984,-90 336.984,-54 551.048,-54 551.048,-90"/><text text-anchor="middle" x="444.016" y="-69" font-family="Courier,monospace" font-size="10.00">&lt;__main__.A object at 0xdeadbeef&gt;</text></g><g id="node2" class="node"><title>struct</title><polygon fill="#eeeeee" stroke="none" points="24,-162 24,-182 256,-182 256,-162 24,-162"/><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-162 24,-182 256,-182 256,-162 24,-162"/><text text-anchor="start" x="27" y="-168.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">struct _object* </text><text text-anchor="start" x="161.422" y="-168.8" font-family="Courier,monospace" font-style="oblique" font-size="14.00" fill="#666666">_ob_next</text><polygon fill="#eeeeee" stroke="none" points="24,-142 24,-162 256,-162 256,-142 24,-142"/><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-142 24,-162 256,-162 256,-142 24,-142"/><text text-anchor="start" x="27" y="-148.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">struct _object* </text><text text-anchor="start" x="161.422" y="-148.8" font-family="Courier,monospace" font-style="oblique" font-size="14.00" fill="#666666">_ob_prev</text><polygon fill="#eeeeee" stroke="none" points="24,-122 24,-142 256,-142 256,-122 24,-122"/><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-122 24,-142 256,-142 256,-122 24,-122"/><text text-anchor="start" x="27" y="-128.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">Py_ssize_t </text><text text-anchor="start" x="119.415" y="-128.8" font-family="Courier,monospace" font-size="14.00">ob_refcnt</text><polygon fill="#eeeeee" stroke="none" points="24,-102 24,-122 256,-122 256,-102 24,-102"/><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-102 24,-122 256,-122 256,-102 24,-102"/><text text-anchor="start" x="26.5815" y="-108.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">struct _typeobject* </text><text text-anchor="start" x="194.609" y="-108.8" font-family="Courier,monospace" font-size="14.00">ob_type</text><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-82 24,-102 256,-102 256,-82 24,-82"/><text text-anchor="start" x="27" y="-88.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">PyMethodDef* </text><text text-anchor="start" x="136.218" y="-88.8" font-family="Courier,monospace" font-size="14.00">m_ml</text><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-62 24,-82 256,-82 256,-62 24,-62"/><text text-anchor="start" x="27" y="-68.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">PyObject* </text><text text-anchor="start" x="111.014" y="-68.8" font-family="Courier,monospace" font-size="14.00">m_self</text><polygon fill="none" stroke="black" stroke-width="0.5" points="24,-42 24,-62 256,-62 256,-42 24,-42"/><text text-anchor="start" x="27" y="-48.8" font-family="Courier,monospace" font-size="14.00" fill="#888888">PyObject* </text><text text-anchor="start" x="111.014" y="-48.8" font-family="Courier,monospace" font-size="14.00">m_module</text></g><g id="edge1" class="edge"><title>struct:f&#45;&gt;obj</title><path fill="none" stroke="black" stroke-width="0.5" d="M257,-72C280.313,-72 305.269,-72 329.087,-72"/><polygon fill="black" stroke="black" stroke-width="0.5" points="329.277,-74.6251 336.777,-72 329.277,-69.3751 329.277,-74.6251"/></g></g></svg>
+
+Fortunately, `m_self` here has the same offset as `im_self` from before, so we
+can just use the same code:
+
+{% highlight pycon %}
+
+>>> ctypes.memmove(id(f) + offset, ctypes.byref(ctypes.py_object(b)), field_size)
+4474703768
+>>> f.__self__ is b
+True
+>>> f
+<built-in method __sizeof__ of B object at 0x10ab4f150>
+
+{% endhighlight %}
 
 ### Dictionary keys