[Python-ideas] 'where' statement in Python? (original) (raw)

Alex Light scialexlight at gmail.com
Wed Jul 21 04:50:29 CEST 2010

Carl M. johnson wrote:

2.) What happens in this case:

x = y given: return "???" Do we just disallow return inside a given? If so, how would the parser know to allow you to do a def inside a given? i think so because unless i am misunderstanding something the only allowed expressions in a 'given' block would be of the type:

a_variable = an_expression

however one way to think of it is that

ans = do_something(a, b, c) given: a = get_a() b = get_b() c = get_c()

is the same as saying

a = get_a() b = get_b() c = get_c() ans = do_something(a, b, c) del a, b, c

which would seem to indicate that a = do_something() given: return "???"

goes to:

return "???" a = do_something()

would be valid (if extremely bad style)

in the end i think its use in that way would be like the use of a GOTO statement in many languages, technically there is no reason it should not be allowed but still prohibited for stylistic reasons.

anyway why would you ever want to do this? it would be like writing

def f(): return return

it is non-nonsensical and obviously an error

On Tue, Jul 20, 2010 at 8:09 PM, Carl M. Johnson < cmjohnson.mailinglist at gmail.com> wrote:

Questions:

1.) It looks like a lot of the complexity of PEP 3150 is based on wanting thing like this to work: x[index] = 42 given: index = complicatedformula To make that work, you need to figure out if index is a nonlocal or a global or what in order to emit the right bytecode. What happens if we just give up that use case and say that anything on the assignment side of the initial = gets looked up in the original namespace? In other words, make 3150 more similar to the sugar: def (): index = complicatedformula x[index] = () #Probably a NameError Would the complexity of PEP 3150 be significantly lessened by that? Or are there other major sources of complexity in the local/nonlocal/global issue? 2.) What happens in this case: x = y given: return "???" Do we just disallow return inside a given? If so, how would the parser know to allow you to do a def inside a given? -- Carl Johnson

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