Improve error message for unbalanced unpacking

[tusharsadhwani]

Feature or enhancement

Proposal:

This is great:

>>> x, y, z = 1, 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: not enough values to unpack (expected 3, got 2)

But here, it doesn't seem to tell how many values were found:

>>> x, y, z = 1, 2, 3, 4
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: too many values to unpack (expected 3)

I was hoping to see (expected 3, got 4) here. The eval loop also seems to have access to an entire PyList object at this point, so getting its length for the error message should not cause any changes in behaviour.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-122239: Add actual count in unbalanced unpacking error message when possible #122244

[HarryLHW]

If the right hand side is an iterator, it will consume more elements of the iterator than before, e.g.

>>> a = iter(range(100))
>>> x, y, z = a
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    x, y, z = a
    ^^^^^^^
ValueError: too many values to unpack (expected 3)
>>> next(a)  # instead of StopIteration
4 
>>> 

[tusharsadhwani]

I see! Would it be possible to only add the length for Sequence / Sized types?

[tusharsadhwani]

Also, isn't the current behaviour also slightly undesirable? the iterator had 4 values consumed from it, instead of 3. Is this behaviour to be depended upon? As in, should programs depend on the behaviour that if an unbalanced unpacking happens, N+1 items will be consumed?

(These are genuine questions, this is my first time trying to reason about the interals of the eval loop.)

[tusharsadhwani]

OK thinking more about this, if it's an infinite iterator you definitely don't want to be stuck forever here.

[Eclips4]

OK thinking more about this, if it's an infinite iterator you definitely don't want to be stuck forever here.

You're right - we need to know how many values are on the right side. But if we have infinity iterator on the right, it will take forever. So, unfortunately, there's no way to do this, even though the idea is good. :( I think we should close this issue.

[tusharsadhwani]

We can't do something like?

if (PySequence_Check(v)) {
    ll = PySequence_Size(v);
    _PyErr_Format(tstate, PyExc_ValueError,
                "too many values to unpack (expected %d, got %d)",
                argcnt, ll);
} else {
    _PyErr_Format(tstate, PyExc_ValueError,
                "too many values to unpack (expected %d)",
                argcnt);
}