Naming things is hard because names represent abstractions. Complex abstractions are hard to get right and hard to summarize; widely-used abstractions and names are hard to change.

A nested function is a simple abstraction in a narrow scope. Both the abstraction and the name are easy to change, so the name only has to describe what the function does now. When that's hard, it's often because the function does too much, does too little, or belongs somewhere else. Likewise, people arguing about the organization of inner functions is usually a sign the outer function should be refactored. Anonymous functions are appropriate when names would be almost redundant, not when coming up with a name is hard.

My editor can show me where a function is used; it can't summarize what a function does. Either way, you have to find where "fooItems" is used to know if you can modify the function.

Conversion functions are very easy to name ("itemToFoo"). Idiomatic Python would just use a loop:

    foo_items = []
    for item in values:
        foo_items.append({
            # foo properties
        })