Python object lookup interview: descriptor precedence before instance state

Operation contract

The receipt object has two descriptor-backed labels. Directly adding a dictionary entry shadows the non-data descriptor but cannot shadow the data descriptor. The program deliberately edits __dict__ to expose lookup order; normal assignment to the read-only data descriptor is rejected. This trace connects properties and methods to the same attribute mechanism.

Failure and ownership boundary

Custom __getattribute__ and __getattr__ hooks can alter behavior, so this fixture covers normal object lookup only. Descriptor lookup is also not a security boundary against code that can directly alter objects. Python descriptors: field validation and attribute precedence and Python staticmethod and classmethod: choose the bound receiver explain when ordinary-looking attribute access runs code.

Working program

class CachedLabel:
    def __get__(self, instance, owner):
        return "class label"

class RequiredLabel:
    def __get__(self, instance, owner):
        return "required label"
    def __set__(self, instance, value):
        raise AttributeError("read-only label")

class ReceiptView:
    cached = CachedLabel()
    required = RequiredLabel()

receipt = ReceiptView()
receipt.__dict__["cached"] = "instance label"
receipt.__dict__["required"] = "attempted shadow"
print(receipt.cached)
print(receipt.required)
try:
    receipt.required = "replacement"
except AttributeError:
    print("normal assignment rejected")

Output

instance label
required label
normal assignment rejected

Costs and limits

Normal lookup examines class/instance state and the inheritance path, with descriptor execution adding its own cost. A descriptor can perform arbitrary I/O or computation; an attribute’s compact spelling does not make it free.

Common Mistakes

• An instance dictionary entry does not override a data descriptor under normal lookup.
• Attribute access can execute a descriptor method.

Connected lessons

Python descriptors: field validation and attribute precedence, Python properties: validate mutation at the public attribute boundary, Python staticmethod and classmethod: choose the bound receiver.