I ran the following program, and found its output surprising in one
place:
class OnlyAl:
def __getitem__(self, key): return 'al'
class OnlyBob(dict):
def __getitem__(self, key): return 'bob'
import sys; print sys.version
al = OnlyAl()
bob = OnlyBob()
print al['whatever']
al.__getitem__ = lambda key: 'NEW AND IMPROVED AL!'
print al['whatever']
print bob['whatever']
bob.__getitem__ = lambda key: 'a NEW AND IMPROVED BOB seems
impossible'
print bob['whatever']
2.6.2 (release26-maint, Apr 19 2009, 01:56:41)
[GCC 4.3.3]
al
NEW AND IMPROVED AL!
bob
bob
In attempting to change the behavior for bob's dictionary lookup, I am
clearly doing something wrong, or maybe even impossible.
Obviously the examples are contrived, but I am interested on a purely
academic level why setting __getitem__ on bob does not seem to change
the behavior of bob['foo']. Note that OnlyBob subclasses dict;
OnlyAl does not.
On a more practical level, I will explain what I am trying to do.
Basically, I am trying to create some code that allows me to spy on
arbitrary objects in a test environment. I want to write a spy()
method that takes an arbitrary object and overrides its implementation
of __getitem__ and friends so that I can see how library code is
invoking the object (with print statements or whatever). Furthermore,
I want spy() to recursively spy on objects that get produced from my
original object. The particular use case is that I am creating a
context for Django templates, and I want to see which objects are
getting rendered, all the way down the tree. It would be pretty easy
to just create a subclass of the context method to spy at the top
level, but I want to recursively spy on all its children, and that is
why I need a monkeypatching approach. The original version had spy
recursively returning proxy/masquerade objects that intercepted
__getitem__ calls, but it becomes brittle when the proxy objects go
off into places like template filters, where I am not prepared to
intercept all calls to the object, and where in some cases it is
impossible to gain control.
Although I am interested in comments on the general problems (spying
on objects, or spying on Django template rendering), I am most
interested in the specific mechanism for changing the __getitem__
method for a subclass on a dictionary. Thanks in advance!
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