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Make ReturnDict support dict union operators on Python 3.9 and later #8302

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Merged
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Dec 22, 2021
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Make ReturnDict support dict union operators on Python 3.9 and later #8302

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17 changes: 17 additions & 0 deletions rest_framework/utils/serializer_helpers.py
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Original file line number Diff line number Diff line change
@@ -1,3 +1,4 @@
import sys
from collections import OrderedDict
from collections.abc import Mapping, MutableMapping

Expand Down Expand Up @@ -28,6 +29,22 @@ def __reduce__(self):
# but preserve the raw data.
return (dict, (dict(self),))

if sys.version_info >= (3, 9):
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Is there a reason for the version gate here?
The methods could be always defined, even if only python >= 3.9 uses them.

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@spookylukey spookylukey Dec 17, 2021 •
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It's not a case of only Python 3.9 using them - no matter what your Python version, if you define those methods, then | will do dictionary merging like it does in 3.9 (I have confirmed this). So the question is - if I'm on Python 3.8, what do I expect these would do:

>>> {} | {}
>>> serializer_instance.data | {}

On Python 3.8, the first one will produce TypeError: unsupported operand type(s) for |: 'dict' and 'dict' and there is nothing we can do about that. So I expect most people would expect the second to do that as well.

I think we are asking for confusion and trouble if we effectively backport PEP 584 to earlier Python versions, but only for the return value of Serializer.data, and nowhere else (because we can't).

# These are basically copied from OrderedDict, with `serializer` added.
def __or__(self, other):
if not isinstance(other, dict):
return NotImplemented
new = self.__class__(self, serializer=self.serializer)
new.update(other)
return new

def __ror__(self, other):
if not isinstance(other, dict):
return NotImplemented
new = self.__class__(other, serializer=self.serializer)
new.update(self)
return new


class ReturnList(list):
"""
Expand Down
22 changes: 22 additions & 0 deletions tests/test_serializer.py
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Original file line number Diff line number Diff line change
Expand Up @@ -740,3 +740,25 @@ class TestSerializer(A, B):
'f4': serializers.CharField,
'f5': serializers.CharField,
}


class Test8301Regression:
@pytest.mark.skipif(
sys.version_info < (3, 9),
reason="dictionary union operator requires Python 3.9 or higher",
)
def test_ReturnDict_merging(self):
# Serializer.data returns ReturnDict, this is essentially a test for that.

class TestSerializer(serializers.Serializer):
char = serializers.CharField()

s = TestSerializer(data={'char': 'x'})
assert s.is_valid()
assert s.data | {} == {'char': 'x'}
assert s.data | {'other': 'y'} == {'char': 'x', 'other': 'y'}
assert {} | s.data == {'char': 'x'}
assert {'other': 'y'} | s.data == {'char': 'x', 'other': 'y'}

assert (s.data | {}).__class__ == s.data.__class__
assert ({} | s.data).__class__ == s.data.__class__