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next on an empty iter raises StopIteration

>>> next(iter([]))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

And if you do this in a list comprehension you get the same error

>>> x, y = [next(iter(z)) for z in ([], [])]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in <listcomp>
StopIteration

But if you change list comprehension to a generator expression then the exception type is transformed!

>>> x, y = (next(iter(z)) for z in ([], []))
Traceback (most recent call last):
  File "<stdin>", line 1, in <genexpr>
StopIteration

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: generator raised StopIteration

This is new behavior as of Python 3.5 https://peps.python.org/pep-0479/