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POPython: How do I dynamically alter methods of dict and list objects?
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  1. POPython: How do I dynamically alter methods of dict and list objects?
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    copied!<p>Here is a mockup of what I want to do:</p> <pre><code>alist = [1,2,3,4,5] # create a vanilla python list object replacef (alist) # replace __setitem__, extend,... with custom functions alist[0]=2 # now the custom __setitem__ is called </code></pre> <p>This is for a DSL project where the syntax should be as close to normal python as possible, so subclassing list and making the user call something like alist = MyList(1,2,3,4,5) is not desirable. Also, because the DSL needs to coexist with other libraries, globally changing list and dict is not an option...</p> <p>I have tried creating instancemethods and setting them directly on the object like alist.append = myFunc, but Python says those properties are read only. Changing the <code>__class__</code> attribute seems to be not permitted as well.</p> <p>Is what I am trying to do even possible in Python?</p> <p><strong>Update</strong>: here are some findings on what is possible with a subclass of object:</p> <p>Given:</p> <pre><code>class y(object): def __init__(self,a,b,c): self.a = a self.b = b self.c = c def f(self): print self print self.a print self.b print self.c class x(object): def __init__(self,a,b,c): self.a = a self.b = b self.c = c def f(self): print "x.f()" &gt;&gt;&gt; objy = y(1,2,3) &gt;&gt;&gt; objx = x(4,5,6) &gt;&gt;&gt; objy.f() &lt;__main__.y object at 0x02612650&gt; 1 2 3 &gt;&gt;&gt; objx.f() x.f() </code></pre> <p>It is possible to change the class of objx on the fly like this:</p> <pre><code>&gt;&gt;&gt; objx.__class__ = y &gt;&gt;&gt; objx.f() &lt;__main__.y object at 0x02612D90&gt; 4 5 6 </code></pre> <p>Also, one can dynamically add/change object methods on the fly, much like in javascript:</p> <pre><code>&gt;&gt;&gt; def g(self,p): print self print p &gt;&gt;&gt; import new &gt;&gt;&gt; objy.g = new.instancemethod(g,objy,y) &gt;&gt;&gt; objy.g(42) &lt;__main__.y object at 0x02612650&gt; 42 </code></pre> <p>However, both approaches will fail with objects that are implemented in C like dict and list:</p> <pre><code>&gt;&gt;&gt; def mypop(self): print "mypop" list.mypop(self) &gt;&gt;&gt; alist.pop = new.instancemethod(mypop,alist,list) AttributeError: 'list' object attribute 'pop' is read-only &gt;&gt;&gt; x = [1,2,3,4,5] &gt;&gt;&gt; x.__class__ = mylist TypeError: __class__ assignment: only for heap types </code></pre> <p>The suggested approach to use something like alist = replacef(alist) would not work here, because replacef could be called from a <code>__setattribute__</code> call like this: </p> <pre><code>alist = [1,2,3,4,5] aDSLObject.items = alist # __setattribute__ calls replacef on alist alist[0] = .... </code></pre> <p>So while it indeed is possible to change object methods dynamically, it seems like it is not possible to alter the behaviour of objects implemented in C - or am I missing something?</p>
 

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