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The _ast.AST constructor requires a tuple with the same number of items than the fields list.
Example with Raise:
ASDL: Raise(expr? exc, expr? cause)
>>> import _ast
>>> _ast.Raise()
<_ast.Raise object at 0x7fc2ca7dee90>
>>> _ast.Raise(1)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: Raise constructor takes either 0 or 2 positional arguments
>>> _ast.Raise(1, 2)
<_ast.Raise object at 0x7fc2ca7def60>
The cause field is optional in the ASDL, but required in the _ast module.
A tradeoff would be to add the minimum and maximum number of fields to _ast.AST.
This issue should prevent API breakage caused by the new docstring attribute added by issue #29463.