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_drafts/python-object-replacement.md

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+---
+layout: post
+title: Replacing Objects in Python
+tags: Python
+description: More reflection than you cared to ask for
+draft: true
+---
+
+Today, we're going to demonstrate a fairly evil thing in Python, which I call
+_object replacement_.
+
+## Review
+
+First, a recap on terminology here. You can skip this if you know Python well.
+
+In Python, _names_ are what most languages call "variables". They reference
+_objects_. So when we do:
+
+{% highlight python %}
+
+a = [1, 2, 3, 4]
+
+{% endhighlight %}
+
+We are creating a list object with four integers, and binding it to the name
+`a`:
+
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="303px" height="53px" version="1.1"><defs/><g transform="translate(0.5,0.5)"><rect x="181" y="6" width="120" height="40" fill="#ffffff" stroke="#000000" pointer-events="none"/><g transform="translate(185,17)"><switch><foreignObject pointer-events="all" width="112" height="20" requiredFeatures="http://www.w3.org/TR/SVG11/feature#Extensibility"><div xmlns="http://www.w3.org/1999/xhtml" style="display: inline-block; font-size: 14px; font-family: Helvetica; color: rgb(0, 0, 0); line-height: 1.26; vertical-align: top; width: 112px; white-space: normal; text-align: center;"><div xmlns="http://www.w3.org/1999/xhtml" style="display:inline-block;text-align:inherit;text-decoration:inherit;"><font face="Courier New">[1, 2, 3, 4]</font></div></div></foreignObject><text x="56" y="17" fill="#000000" text-anchor="middle" font-size="14px" font-family="Helvetica">[Not supported by viewer]</text></switch></g><ellipse cx="26" cy="26" rx="25" ry="25" fill="#ffffff" stroke="#000000" pointer-events="none"/><g transform="translate(13,11)"><switch><foreignObject pointer-events="all" width="26" height="32" requiredFeatures="http://www.w3.org/TR/SVG11/feature#Extensibility"><div xmlns="http://www.w3.org/1999/xhtml" style="display: inline-block; font-size: 24px; font-family: 'Courier New'; color: rgb(0, 0, 0); line-height: 1.26; vertical-align: top; width: 26px; white-space: normal; text-align: center;"><div xmlns="http://www.w3.org/1999/xhtml" style="display:inline-block;text-align:inherit;text-decoration:inherit;">a</div></div></foreignObject><text x="13" y="28" fill="#000000" text-anchor="middle" font-size="24px" font-family="Courier New">[Not supported by viewer]</text></switch></g><path d="M 51 26 L 175 26" fill="none" stroke="#000000" stroke-miterlimit="10" pointer-events="none"/><path d="M 180 26 L 173 30 L 175 26 L 173 23 Z" fill="#000000" stroke="#000000" stroke-miterlimit="10" pointer-events="none"/></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
+memory address (which you can get using `id(a)`, but that's a CPython
+implementation detail).
+
+{% highlight python %}
+
+b = a
+
+{% endhighlight %}
+
+{% highlight python %}
+
+c = SomeContainerClass()
+c.data = a
+
+{% endhighlight %}
+
+{% highlight python %}
+
+def wrapper(L):
+    def inner():
+        return L.pop()
+    return inner
+
+d = wrapper(a)
+
+{% endhighlight %}
+
+[insert charts here]
+
+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` (and
+`d.func_closure[0].cell_contents` from the perspective of the outside world,
+even though that's cumbersome and you would never do that in practice). As a
+result, if you delete one of these references—explicitly with `del a`, or
+implicitly if a name goes out of scope—then the other references are still
+around, and object continues to exist. If all of an object's references
+disappear, then Python's garbage collector will eliminate it.
+
+## Fishing for references with Guppy
+
+[...]
+
+So, what this boils down to is finding all references to a given object and
+replacing them with another object.