Is there a way to insert hooks into a native dictionary type to see when a query arrives and what's looked up?

Veek M vek.m1234 at gmail.com
Fri Dec 16 23:42:28 EST 2016


Steven D'Aprano wrote:

> On Wednesday 14 December 2016 17:11, Veek M wrote:
> 
>> I know that with user classes one can define getattr, setattr to
>> handle dictionary lookup. Is there a way to hook into the native
>> dict() type and see in real time what's being queried.
> 
> Not easily, and maybe not at all.
> 
> There are two obvious ways to do this:
> 
> (1) monkey-patch the object's __dict__, and the class __dict__.
> 
> Unfortunately, Python doesn't support monkey-patching built-ins.
> 
> https://en.wikipedia.org/wiki/Monkey_patch
> 
> Or perhaps I should say, *fortunately* Python doesn't support it.
> 
> http://www.virtuouscode.com/2008/02/23/why-monkeypatching-is-destroying-ruby/
> 
> (2) Alternatively, you could make a dict subclass, and replace the
> class and instance __dict__ with your own.
> 
> Unfortunately, you cannot replace the __dict__ of a class:
> 
> py> class X:  # the class you want to hook into
> ...     pass
> ...
> py> class MyDict(dict):  # my custom dict
> ...     def __getitem__(self, key):
> ...             print(key)
> ...             return super().__getitem__(key)
> ...
> py> d = MyDict()
> py> d.update(X.__dict__)
> py> X.__dict__ = d
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> AttributeError: attribute '__dict__' of 'type' objects is not writable
> 
> 
> You can replace the instance dict, but Python won't call your
> __getitem__ method:
> 
> py> instance = X()
> py> instance.__dict__ = MyDict()
> py> instance.a = 999
> py> instance.a
> 999
> 
> So the short answer is, No.
> 
> You might be able to create a completely new metaclass that supports
> this, but it would be a lot of work, and I'm not even sure that it
> would be successful.
> 
> 
> 
>> I wanted to check if when one does:
>> 
>> x.sin()
>> 
>> if the x.__dict__ was queried or if the Foo.__dict__ was queried..
> 
> The easiest way to do that is something like this:
> 
> 
> py> class Test:
> ...     def sin(self):
> ...             return 999
> ...
> py> x = Test()
> py> x.sin
> <bound method Test.sin of <__main__.Test object at 0xb6fc3a4c>>
> py> x.sin()
> 999
> py> x.sin = "surprise!"
> py> x.sin
> 'surprise!'
> 
> 
> 
> So now you know: an instance attribute will shadow the class
> attribute.
> 
> (Actually, that's not *completely* true. It depends on whether x.sin
> is a descriptor or not, and if so, what kind of descriptor.)
> 
> 

heh
If it walks like a duck and talks like a duck, it’s a duck, right? So if 
this duck is not giving you the noise that you want, you’ve got to just 
punch that duck until it returns what you expect. -Patrick Ewing on 
Monkey/Duck patching in RailsConf 2007


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