TypeError: unsupported operand type(s)

Hi there. I have an edge-case where pyupgrade suggested breaking changes to my code, which I think is because it assumes that any usage of ** implies that it’s a dictionary and can be |'d with other dictionaries. I’m running pyupgrade with --py39-plus.

from collections.abc import Mapping


class A(Mapping):
    proxy = {"foo": 1}

    def __getitem__(self, k):
        return self.proxy.__getitem__(k)

    def __len__(self):
        return self.proxy.__len__()

    def __iter__(self):
        return self.proxy.__iter__()

a = A()
b = {"bar": 2}

# Basic assumption holds.
assert {**a} == {"foo": 1}

# Original code works.
assert {**a, **b} == {"foo": 1, "bar": 2}

# Pyupgrade suggestion
assert a | b == {"foo": 1, "bar": 2}
# TypeError: unsupported operand type(s) for |: 'A' and 'dict'

Is this an edge-case that Pyupgrade should handle, or is it safe to assume that any class that uses ** needs to implement compatibility with a dictionary union?

[MarcoGorelli]

which I think is because it assumes that any usage of ** implies that it’s a dictionary

the code that deals with this is here

https://github.com/asottile/pyupgrade/blob/aaad686b5f7851dd1b269f87cb5832ed132f4631/pyupgrade/_plugins/pep584.py

If it thinks it’s a dictionary, it’s because that’s what’s detected by the inbuilt ast module

Running astpretty on your code snippet, we get

                left=Dict(
                    lineno=29,
                    col_offset=7,
                    end_lineno=29,
                    end_col_offset=17,
                    keys=[
                        None,
                        None,
                    ],
                    values=[
                        Name(lineno=29, col_offset=10, end_lineno=29, end_col_offset=11, id='a', ctx=Load()),
                        Name(lineno=29, col_offset=15, end_lineno=29, end_col_offset=16, id='b', ctx=Load()),
                    ],
                ),

for {**a, **b}

[asottile]

@spookylukey please don’t mention that project here – the maintainer is not a kind member of open source and I do not want to give them free advertising