Confusion between class variable and instance variable when descriptor is used

[danabr]

Prerequisites

• Are you running the latest version?
• Are you reporting to the correct repository?
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Description

Consider the following program:

class ListDescriptor:
    def __set__(self, obj, value):
        obj.__dict__["x"] = value

class Test:
    x = ListDescriptor()

    def __init__(self):
        self.x = [1,2,3]

t = Test()
print(t.x)

The output from this program differs between CPython and IronPython.

Steps to Reproduce

1. Run the above program

Expected behavior: Output should be [1, 2, 3].

Actual behavior: Output is <ListDescriptor object at 0x000000000000002B>.

Workarounds

You can work around this by explictly defining the __get__ method in ListDescriptor. However, we encountered this in third-party code that we would rather not modify if it could be avoided.

[slozier]

Note that there's already a test case that could be enabled which would cover this test_descr.test_set_and_no_get.