How can I access attributes of a lazy variable class?

Question

I've made myself a lazy variable class, and used it in another class. How can I then access the attributes of the lazy variable class? I have tried __getattr__ without luck. Here's an example:

class lazyobject(object):
    def __init__(self,varname,something='This is the something I want to access'):
        self.varname = varname
        self.something = something

    def __get__(self, obj, type=None):
        if obj.__dict__.has_key(self.varname):
            print "Already computed %s" % self.varname
            return obj.__dict__[self.varname]
        else:
            print "computing %s" % self.varname
            obj.__dict__[self.varname] = "something else"
            return obj.__dict__[self.varname]

class lazyobject2(lazyobject):
    def __getattr__(self):
        return self.something

class dummy(object):
    def __init__(self):
        setattr(self.__class__, 'lazy', lazyobject('lazy'))

class dummy2(object):
    def __init__(self):
        setattr(self.__class__, 'lazy', lazyobject2('lazy'))

d1 = dummy()
d2 = dummy2()

try:
    print "d1.lazy.something - no getattr: ",d1.lazy.something
except:
    print "d2.lazy is already computed - can't get its .something because it's now a string!"
print "d1.lazy - no getattr: ",d1.lazy

try:
    print "d2.lazy.something - has getattr: ",d2.lazy.something
except:
    print "d2.lazy is already computed - can't get its .something because it's now a string!"
print "d2.lazy - no getattr: ",d2.lazy

This prints:

d1.lazy.something - no getattr:  computing lazy
d2.lazy is already computed - can't get its .something because it's now a string!
d1.lazy - no getattr:  something else
d2.lazy.something - has getattr:  computing lazy
d2.lazy is already computed - can't get its .something because it's now a string!
d2.lazy - no getattr:  something else

What I would like it to print:

d1.lazy.something - no getattr:  This is the something I want to access
computing lazy
d1.lazy - no getattr:  something else

The above example is contrived but I hope gets the point across. Another way to phrase my question is: How can I bypass the __get__ method when accessing a class attribute?

Raymond Hettinger · Accepted Answer · 2011-11-08 04:31:34Z

The way to bypass __get__ when accessing a class attribute is to look it up via the class dictionary rather than using dotted access.

This is easy to demonstrate using function objects. For example:

>>> class A(object):
        def f(self):
            pass

>>> A.f                         # dotted access calls f.__get__
<unbound method A.f>
>>> vars(A)['f']                # dict access bypasses f.__get__
<function f at 0x101723500>

>>> a = A()
>>> a.f                         # dotted access calls f.__get__
<bound method A.f of <__main__.A object at 0x10171e810>>
>>> vars(a.__class__)['f']      # dict access bypasses f.__get__
<function f at 0x101723500>

The other piece of information you were missing is that the inherited __get__ runs before the __getattr__ which only runs if no attribute is found. This logic is controlled by __getattribute__ which is inherited from object. So, if you want to bypass __get__ you will either need to write a new __get__ in the subclass or change the lookup logic by defining __getattribute__ in the subclass.

To fix the lazyobject2 class, replace the __getattr__ with:

class lazyobject2(lazyobject):

    def __getattribute__(self, key):
        # bypass __get__
        return object.__getattribute__(self, '__dict__')[key]

In summary, the key pieces of knowledge used to solve this problem are:

object.__getattribute__ controls the lookup logic.
It first looks for __get__ whether defined in the current class or inherited.
Only if nothing is found, does it attempt to call object.__getattr__.
The above three steps only happen for dotted lookup.
Those step can be bypassed by directly accessing the dict via __dict__ or vars().

The full details of descriptor logic can be found in this writeup or in this presentation.

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