Implemented "new" keyword for a JSFunction

[desertkun]

So you can do this:

from v8py import Context, new

ctx = Context()
ctx.eval("""
    function Test(who) { this.who = who; }
    Test.prototype.test = function() { return "Hello, " + this.who + "!"; }
""")

# Test is a JSFunction here
Test = ctx.glob.Test 

# Javascript equivalent of instance = new Test("world")
instance = new(Test, "world") 

test = instance.test()
print test

> Hello, world!

[codecov-io]

Codecov Report

Merging #14 into master will increase coverage by 3.3%. The diff coverage is 88.23%.

@@           Coverage Diff            @@
##           master     #14     +/-   ##
========================================
+ Coverage   67.79%   71.1%   +3.3%     
========================================
  Files          16      16             
  Lines        1180    1263     +83     
  Branches      184     205     +21     
========================================
+ Hits          800     898     +98     
+ Misses        257     237     -20     
- Partials      123     128      +5

Impacted Files	Coverage Δ
v8py/jsfunction.cpp	96% <88.23%> (+51.55%)	:arrow_up:
v8py/convert.cpp	67.97% <0%> (+1.96%)	:arrow_up:
v8py/jsobject.cpp	78.65% <0%> (+4.49%)	:arrow_up:
v8py/v8py.cpp	75.88% <0%> (+7.98%)	:arrow_up:
v8py/polyfill.h	100% <0%> (+25%)	:arrow_up:

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[tbodt]

I don't like the idea of adding a method to JSFunction or JSObject. What if JavaScript sets Test.new to something?

I'd prefer just being able to say Test() and have it use the new operator if you're working with a class. This isn't generally possible to find out, though. We could check whether there is a property called prototype, but you can use new with functions that don't have prototypes. There's a v8::Object::IsConstructor method that seems promising, but I'm pretty sure it returns true for every user-defined function (since you can use new on those).

Since that won't work, the interface I'd prefer is something like this:

from v8py import new
instance = new(Test, "world")

or maybe

instance = new(Test)("world")

[desertkun]

Yeah, good point.

What's next? I can add a new commit into my bound_function_new that changes the API into new(Test, "world") OR, we can dismiss this pull request and start a new one, can we?

[tbodt]

You could do either.

[desertkun]

Okay, changed it to be context.new(Test, "world") since context is required. I tried to pull one from Test, but context is also required to do get Local<Value> of the constructor function in the first place.

Also updated tests.

[tbodt]

The reason a context is required by js_from_py is it has to be able to handle Python objects as well as JavaScript objects, but new doesn't need to handle Python objects. In fact, it can throw an exception if the argument isn't a JSFunction. So you can just do PyArg_ParseTuple(args, "O!", &js_function_type, &function). That also takes care of throwing a TypeError if it's the wrong type.

[tbodt]

Could you implement this as a function on the v8py module?

[desertkun]

Well, I was going to, but came into a question: what if you need to spawn a new object into context A from a constructor function that belongs to context B? How the new new can handle that? I came into conclusion that explicitly specifying the context would solve the problem.

I can get rid of js_from_py, though, and make it to use js_function_type like you said, but I have to specify context anyway for the very reason above. Well, I can make function new(constructor, *args, context=None) of the v8py module so you can specify one if you want but that's not obvious implementation and this project does lack documentation. So I am worried would someone ever find this feature?

I also can additionally add a new function into a module that simply uses the context of the constructor function in order to spawn an object. Are you okay with two implementations, one for explicitly specifying target context, and one without such?

What do you say?

[tbodt]

V8 won't let you call a function or constructor from one context in a different context, the execution always happens in the context where the function was created. You can then put the resulting object in any context you like.

[desertkun]

Please take my apology for taking so much of your time regarding this issue.

Finally, moved the method to the function to the v8py module.

[tbodt]

Please take my apology for taking so much of your time regarding this issue.

Oh, it's no problem. Not sure if I should be apologizing for making you rewrite this 15 times :)