[Python-Dev] 2.5 and beyond (original) (raw)

Greg Ewing greg.ewing at canterbury.ac.nz
Mon Jul 3 03:50:54 CEST 2006

Tim Peters wrote:

Scheme has no loops in Python's sense -- things like "do" are shorthand for expressing stylized recursion

But it does have foreach and map, which are the moral equivalent of Python's for-loops and list comprehensions. The body is a lambda which takes the loop variable as a parameter, thus providing the extra level of scope. Recursion isn't needed.

When people talk about giving a Python for-loop vrbl its own scope, they generally don't mean a new scope on each iteration,

But that's exactly what you do need in order for a for-loop with a lambda in it to behave intuitively. If that's not what people mean, it's because they don't fully understand what they really mean to mean. :-)

BTW, I'm not suggesting that a new stack frame gets allocated for every iteration -- all you need is a cell.

about the same as creating a nested block with its own autos in C.

Analogies with C aren't very helpful here, because it doesn't have closures, so it's only a matter of visibility, not lifetime.

creating a new scope on each iteration sounds hard to explain in Python.

But is it harder to explain than the reason someone's loop-with-a-lambda doesn't do what they expect?

BTW, I wouldn't explain it by saying it creates a new scope, I'd say it creates a new binding on each iteration, or something like that.

In my earlier proposal, you would actually say that explicitly, with something like

for new i in range(10): ...

Abusing the default-argument machinery to capture current bindings is never necessary, and is abuse.

But it's abuse that still happens, because although scoping has been fixed, other parts of the story are still missing.

-- Greg

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