Intercepting __getitem__ calls on an object attribute

Question

Question: How can I Intercept __getitem__ calls on an object attribute?

Explanation:

So, the scenario is the following. I have an object that stores a dict-like object as an attribute. Every time the __getitem__ method of this attribute gets called, I want to intercept that call and do some special processing on the fetched item depending on the key. What I want would look something like this:

class Test:

    def __init__(self):
        self._d = {'a': 1, 'b': 2}

    @property
    def d(self, key):
        val = self._d[key]
        if key == 'a':
            val += 2
        return val
t = Test()
assert(t.d['a'] == 3) # Should not throw AssertionError

The problem is that the @property method doesn't actually have access to the key in the __getitem__ call, so I can't check for it at all to do my special postprocessing step.

Important Note: I can't just subclass a MutableMapping, override the __getitem__ method of my subclass to do this special processing, and store an instance of the subclass in self._d. In my actual code self._d is already a subclass of MutableMapping and other clients of this subclass need access to the unmodified data.

Thanks for any and all help!

Answer 1

One solution would be a Mapping that proxies the underlying mapping. The d property would wrap the underlying self._d mapping in the proxy wrapper and return it, and use of that proxy would exhibit the necessary behaviors. Example:

from collections.abc import Mapping

class DProxy(Mapping):
    __slots__ = ('proxymap',)
    def __init__(self, proxymap):
        self.proxymap = proxymap
    def __getitem__(self, key):
        val = self.proxymap[key]
        if key == 'a':
            val += 2
        return val
    def __iter__(self):
        return iter(self.proxymap)
    def __len__(self):
        return len(self.proxymap)

Once you've made that, your original class can be:

class Test:
    def __init__(self):
        self._d = {'a': 1, 'b': 2}

    @property
    def d(self):
        return DProxy(self._d)

Users would then access instances of Test with test.d[somekey]; test.d would return the proxy, which would then modify the result of __getitem__ as needed for somekey. They could even store off references with locald = test.d and then use locald while preserving the necessary proxy behaviors. You can make it a MutableMapping if needed, but a plain Mapping-based proxy avoids complexity when the goal is reading the values, never modifying them through the proxy.

Yes, this makes a new DProxy instance on each access to d; you could cache it if you like, but given how simple the DProxy class's __init__ is, the cost is only meaningful if qualified access via the d attribute is performed frequently on the hottest of code paths.