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Currently, if you calldataclasses.fields on a non-dataclass, quite a poor traceback is produced:
>>>import dataclasses>>> dataclasses.fields(object)Traceback (most recent call last): File "C:\Users\alexw\AppData\Local\Programs\Python\Python311\Lib\dataclasses.py", line 1232, in fields fields = getattr(class_or_instance, _FIELDS) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^AttributeError: type object 'object' has no attribute '__dataclass_fields__'During handling of the above exception, another exception occurred:Traceback (most recent call last): File "<stdin>", line 1, in <module> File "C:\Users\alexw\AppData\Local\Programs\Python\Python311\Lib\dataclasses.py", line 1234, in fields raise TypeError('must be called with a dataclass type or instance')TypeError: must be called with a dataclass type or instance
There are two issues here:
- The traceback mentions the
__dataclass_fields__attribute, which is an internal implementation detail of thedataclassesmodule and should be hidden from the user. - The "during handling of the above exception, another exception occurred" message implies to the user that there's a bug in the
dataclassesmodule itself, rather than this being intentional behaviour.
This one-line change todataclasses.py produces a much better traceback:
diff --git a/Lib/dataclasses.py b/Lib/dataclasses.pyindex 82b08fc017..e3fd0b3e38 100644--- a/Lib/dataclasses.py+++ b/Lib/dataclasses.py@@ -1248,7 +1248,7 @@ def fields(class_or_instance): try: fields = getattr(class_or_instance, _FIELDS) except AttributeError:- raise TypeError('must be called with a dataclass type or instance')+ raise TypeError('must be called with a dataclass type or instance') from None
With this change applied, we get this traceback instead:
>>>import dataclasses>>> dataclasses.fields(object)Traceback (most recent call last): File "<stdin>", line 1, in <module> File "C:\Users\alexw\coding\cpython\Lib\dataclasses.py", line 1251, in fields raise TypeError('must be called with a dataclass type or instance') from NoneTypeError: must be called with a dataclass type or instance
Cc.@ericvsmith and@carljm, asdataclasses experts.