gh-116436: Improve error message when TypeError occurs during dict update

[hauntsaninja]

The example I gave in the issue now looks like:

Traceback (most recent call last):
  File "/Users/shantanu/dev/cpython/z.py", line 8, in <module>
    x.update([foo() for _ in range(3)])
    ~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/shantanu/dev/cpython/z.py", line 4, in foo
    yield random.randint(100)  # oops, only passed in one argument
          ~~~~~~~~~~~~~~^^^^^
TypeError: Random.randint() missing 1 required positional argument: 'b'
Cannot convert dictionary update sequence element #0 to a sequence

Previously it was just:

Traceback (most recent call last):
  File "/Users/shantanu/dev/cpython/z.py", line 7, in <module>
    dict([foo() for _ in range(3)])
TypeError: cannot convert dictionary update sequence element #0 to a sequence

• Issue: Poor error message when TypeError is raised during dict construction #116436

[hauntsaninja]

Hopefully this is a good use of exception notes, since it's just adding context. If not, maybe something with PyException_SetCause works

[hauntsaninja]

Technically, I guess PyObject_GetIter has a different error message for the edge case where __iter__ exists but returns a non-iterator.

I think nicest thing would be to change PySequence_Fast so that if m is NULL it doesn't clobber the message

[python-cla-bot]

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