[swift-evolution] Python Interop with Swift 4+

[Paul Cantrell]

> On Nov 20, 2017, at 7:08 AM, David Hart via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
> 
>> On 20 Nov 2017, at 12:34, Brent Royal-Gordon <brent at architechies.com <mailto:brent at architechies.com>> wrote:
>> 
>>> On Nov 20, 2017, at 12:32 AM, David Waite via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>>> On Nov 20, 2017, at 1:16 AM, David Hart via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>> <snip>
>>> 
>>>> Moreover, Ruby allows classes to have instance variables with the same name as methods:
>>>> 
>>>> class Foo
>>>>   def initialize()
>>>>     @bar = 5
>>>>   end
>>>> 
>>>>   def bar()
>>>>     puts “Hello"
>>>>   end
>>>> end
>>>> 
>>>> In that case, how would one implement DynamicMemberLookupProtocol for the lookup of bar, and what would the return value be? Its entirely context sensitive.
>>> 
>>> I do not believe Ruby does not give you direct external access to variables. Everything with a ‘.’ is a function call.
>>> 
>>> You would use e.g.
>>> 
>>> Foo.new.instance_variable_get("@bar”) // => 5
>>> 
>>> attr :bar exposes a variable @bar through functions bar() and bar=()   (and also optimizes storage in some implementations)
>> 
>> This is correct…sort of. Ruby uses symbols to refer to both methods and instance variables, but instance variable symbols always start with @, so they effectively belong to a different namespace. (Similarly, symbols starting with a capital letter are for constants; symbols starting with @@ are for class variables; I believe symbols starting with $ are for global variables.) Ruby only provides syntax to access another object's methods and (for a class) constants, so in practice there's no way to access another object's instance variables except by calling a method on it, but there's no particular reason our bridge would need to follow that rule.

Just to add a little weight to what Brent wrote here: in Ruby, one can •only• access instance variables of self using the @bar syntax. There is no such thing as “foo. at bar”; as Brent wrote, one can only access another object — not even another class, but another object! — via its methods.

Class authors thus expect @bar to be private (well, same-instance protected), and instance_variable_get/set is the accepted in-Ruby hack to fiddle with another object’s internal state. I see no reason a bridge would need to deviate from that understanding.

>> 
>> Leaving aside those technicalities, it's pretty clear that `foo.bar` should access a method, not an instance variabl, when `foo` is a Ruby object. That doesn't mean it's the same as Python, though, because in Ruby it will need to *call* the method immediately if we're to provide natural syntax for Ruby objects bridged to Swift.
> 
> Exactly. My example was a bit contrived but that’s what I wanted to say.

X-ref to a thread where we discussed this at greater length:

https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20171113/041447.html <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20171113/041447.html>

Cheers,

Paul
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