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When default is None, we know what types are serialisable.
Thus,
import json
import types
import typing
def x(data: typing.Mapping[str, str]) -> str:
return json.dumps(data)
def y():
return json.dumps(types.MappingProxyType({}))Shouldn't type-check.
Running this fails with:
File /opt/homebrew/Cellar/python@3.13/3.13.2/Frameworks/Python.framework/Versions/3.13/lib/python3.13/json/encoder.py:180, in JSONEncoder.default(self, o)
161 def default(self, o):
162 """Implement this method in a subclass such that it returns
163 a serializable object for ``o``, or calls the base implementation
164 (to raise a ``TypeError``).
(...) 178
179 """
--> 180 raise TypeError(f'Object of type {o.__class__.__name__} '
181 f'is not JSON serializable')
TypeError: Object of type mappingproxy is not JSON serializable
Type-checking JSONEncoder itself would be much harder, but the more-commonly-used functions should be doable via overloads.
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Type checkers do not report an error, but shouldType checkers do not report an error, but should