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POWhy is __init__ not called after __new__ SOMETIMES
 

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  1. POWhy is __init__ not called after __new__ SOMETIMES
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    <p>Let me start with this is not a repeat of <a href="https://stackoverflow.com/questions/6903355/why-does-init-not-get-called-if-new-called-with-no-args">Why does __init__ not get called if __new__ called with no args</a>. I have tried to carefully construct some sample code for <code>__new__</code> and <code>__init__</code> that has no explanation I can find.</p> <p>The basic parameters:</p> <ul> <li>There is a base class called NotMine as it comes from another library (I'll disclose at the end, not important here)</li> <li>That class has an <code>__init__</code> method that in turn calls a <code>_parse</code> method</li> <li>I need to override the <code>_parse</code> method in subclasses</li> <li>which subclass I'm creating is not known until invocation</li> <li>I know there are factory design methods but I cannot use them here (More at the end)</li> <li>I have tried to make careful use of <code>super</code> to avoid the problems in <a href="https://stackoverflow.com/questions/6032053/python-logging-why-is-init-called-twice">Python logging: Why is __init__ called twice?</a></li> <li>I know this is also 'kind of' an AbstractBaseMehtod opportunity but that did not help</li> </ul> <p>Anyway, <code>__init__</code> should be called after <code>__new__</code> and for every explanation of why SOME samples below don't work I seem to be able to point to other cases that do work and rule out the explanation.</p> <pre><code>class NotMine(object): def __init__(self, *args, **kwargs): print "NotMine __init__" self._parse() def _parse(self): print "NotMine _parse" class ABC(NotMine): def __new__(cls,name,*args, **kwargs): print "-"*80 print "Entered through the front door ABC.__new__(%s,%s,*%s,**%s)"%(cls,name,args,kwargs) if name == 'AA': obj = super(NotMine,ABC).__new__(AA,*args,**kwargs) print "Exiting door number 1 with an instance of: %s"%type(obj) return obj elif name == 'BB': obj = super(NotMine,ABC).__new__(BB,*args,**kwargs) print "Exiting door number 2 with an instance of: %s"%type(obj) return obj else: obj = super(NotMine,ABC).__new__(cls,*args,**kwargs) print "Exiting door number 3 with an instance of: %s"%type(obj) return obj class AA(ABC): def _parse(self): print "AA _parse" class BB(ABC): def __init__(self, *args, **kw): print "BB_init:*%s, **%s"%(args,kw) super(BB,self).__init__(self,*args,**kw) def _parse(self): print "BB _parse" class CCC(AA): def _parse(self): print "CCCC _parse" print("########### Starting with ABC always calls __init__ ############") ABC("AA") # case 1 ABC("BB") # case 2 ABC("NOT_AA_OR_BB") # case 3 print("########### These also all call __init__ ############") AA("AA") # case 4 BB("BB") # case 5 AA("NOT_AA_OR_BB") # case 6 BB("NOT_AA_OR_BB") # case 7 CCC("ANYTHING") # case 8 print("########### WHY DO THESE NOT CALL __init__ ############") AA("BB") # case 9 BB("AA") # case 10 CCC("BB") # case 11 </code></pre> <p>If you execute the code, you can see that for each call to <code>__new__</code> it announces "which door" it is exiting through and with what type. I can exit the same "door" with the same "type" object and have <code>__init__</code> called in one case and not the other. I've looked at the mro of the "calling" class and that offers no insight since I can invoke that class ( or a subcass as in CCC ) and have <code>__init__</code> called.</p> <p><strong>End Notes:</strong> The <code>NotMine</code> library I'm using is the <a href="http://genshi.edgewall.org/" rel="nofollow noreferrer">Genshi</a> MarkupTemplate and the reason for not using a Factory design method is that their TemplateLoader needs a defaultClass to construct. I don't know until I start parsing, which I do in <code>__new__</code>. There is a lot of cool voodoo magic that genshi loaders and templates do that make this worth the effort. </p> <p>I can run an unmodified instance of their loader and currently everything works as long as I ONLY pass the ABC (abstract sort-of-factory) class as the default. Things are working well but this unexplained behavior is an almost certain bug later. </p> <p><strong>UPDATE:</strong> Ignacio, nailed the top line question, if the returned object is not an "instance of" <em>cls</em> then <code>__init__</code> is not called. I do find that calling the "constructor" (e.g. <code>AA(args..)</code> is wrong as it will call <code>__new__</code> again and you are right back where you started. You could modify an arg to take a different path. That just means you call <code>ABC.__new__</code> twice rather than infinitely. A working solution is to edit <code>class ABC</code> above as:</p> <pre><code>class ABC(NotMine): def __new__(cls,name,*args, **kwargs): print "-"*80 print "Entered through the front door ABC.__new__(%s,%s,*%s,**%s)"%(cls,name,args,kwargs) if name == 'AA': obj = super(NotMine,ABC).__new__(AA,*args,**kwargs) print "Exiting door number 1 with an instance of: %s"%type(obj) elif name == 'BB': obj = super(NotMine,ABC).__new__(BB,*args,**kwargs) print "Exiting door number 2 with an instance of: %s"%type(obj) elif name == 'CCC': obj = super(NotMine,ABC).__new__(CCC,*args,**kwargs) print "Exiting door number 3 with an instance of: %s"%type(obj) else: obj = super(NotMine,ABC).__new__(cls,*args,**kwargs) print "Exiting door number 4 with an instance of: %s"%type(obj) ## Addition to decide who calls __init__ ## if isinstance(obj,cls): print "this IS an instance of %s So call your own dam __init__"%cls return obj print "this is NOT an instance of %s So __new__ will call __init__ for you"%cls obj.__init__(name,*args, **kwargs) return obj print("########### now, these DO CALL __init__ ############") AA("BB") # case 9 BB("AA") # case 10 CCC("BB") # case 11 </code></pre> <p>Notice the last few lines. Not calling <code>__init__</code> if it's a "different" class does not make sense to me, ESPECIALLY when the "different" class is still a subclass of the class calling <code>__init__</code>. I don't like the above edit but least I get the rules a little better now.</p>
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    1. USPhil Cooper
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    1. COIs Genshi using a metaclass? See http://stackoverflow.com/questions/100003/what-is-a-metaclass-in-python
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    2. CONo, My sample code does not use the genshi as a base.
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