TIL: List comprehensions can't see exec() locals in Python < 3.12

[Written by Claude Opus 4.6; reviewed and approved by Saul]

Today we discovered why a VisiData command worked on Python 3.12 but crashed with NameError on Python 3.9.

The command

FreqTableSheet.addCommand('zEnter', 'dive-except',
    'vd.push(openRows([r for r in rows if r is not cursorRow]))',
    'open source sheet excluding current bin')

VisiData commands are strings that get exec()'d, with the sheet's attributes available as locals via a custom mapping.
Both rows and cursorRow are properties on the sheet.

On Python 3.12+, this works fine.
On Python 3.9, it raises NameError: name 'cursorRow' is not defined.

Why

In Python < 3.12, list comprehensions create their own scope — they compile to an implicit nested function.
The exec() locals dict is not a real closure scope, so the comprehension's inner function can't see it.

Here's the minimal reproduction:

d = {'items': [1, 2, 3], 'target': 2}
exec('[x for x in items if x != target]', {}, d)
# Python 3.9:  NameError: name 'target' is not defined
# Python 3.12: [1, 3]

The subtle part: items works because the iterable expression in for x in items is evaluated in the outer scope (the exec context).
But target in the if clause runs inside the comprehension's implicit function, where exec locals are invisible.

PEP 709 (Python 3.12) inlined comprehensions, eliminating the nested scope entirely. So on 3.12+, everything in a comprehension shares the enclosing scope, and the problem disappears.

The fix

Extract the comprehension into a real method, where normal Python scoping rules apply:

@Sheet.api
def exceptRows(sheet, exceptrows):
    if not isinstance(exceptrows, list):
        exceptrows = [exceptrows]
    return [r for r in sheet.rows if r not in exceptrows]

FreqTableSheet.addCommand('zEnter', 'dive-except',
    'vd.push(openRows(exceptRows(cursorRow)))',
    'open source sheet excluding current bin')

Now cursorRow is evaluated in the exec scope (where it's visible), and the comprehension runs inside a normal method with normal scoping.

The lesson

If you use exec() with a locals dict and need to support Python < 3.12: never reference locals inside a comprehension filter or body.
The iterable (for x in thing) is safe; everything else is not.

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