I Challenge You to Debug These 7 Lines of Code Under 9 Minutes

Post by Oxitendwe » Tue, Feb 28, 2017, 7:10 PM UTC

pmarreck wrote: post #30

The fact that "methods" is somehow kept defined in the scope of the function and preserved when you call it again and isn't always reinitialized to [] is just dumb. No wonder this was a bug, who the hell would see that and how is this "feature" ever useful except to create bugs?

Post by zaptheimpaler » Tue, Feb 28, 2017, 7:10 PM UTC

Matthias247 wrote: post #18

lalaithion wrote: post #7

Earlier quoted context omitted.

Basically it just boils down to reference vs value. Both of the errors in this code occur because python uses a reference instead of a value – I agree that they can be annoying and somewhat unexpected, but they make sense once you realize what's behind them. In the method=[] part, the error occurs because python creates a single list, and passes that list by reference (as all lists are passed) into the function. In t…

I understand the "capturing i in a closure by reference" part, since that error is also common in other languages which capture variables by reference, and don't create a new reference per loop iteration. However the default parameter issue looks like a python specific issue which is really strangely designed. I would normally say [] is a literal for an empty list which creates a new instance of an empty list every t…

Post by pmarreck » Tue, Feb 28, 2017, 9:40 PM UTC

Oxitendwe wrote: post #31

pmarreck wrote: post #30

The fact that "methods" is somehow kept defined in the scope of the function and preserved when you call it again and isn't always reinitialized to [] is just dumb. No wonder this was a bug, who the hell would see that and how is this "feature" ever useful except to create bugs?

It's absolutely insane that this is apparently the expected behavior. It makes no sense whatsoever except to give people who know this an ego boost when they can successfully work around Python's unintuitive semantics while inexperienced people cannot. It completely breaks functions, the most fundamental unit of encapsulation and abstraction in every modern programming language.

Post by zaptheimpaler » Thu, Mar 02, 2017, 10:08 AM UTC

jVint wrote: post #36

zaptheimpaler wrote: post #33

Earlier quoted context omitted.

>Is [] is global reference to a single mutable list which is only initially empty? No, the problem is default arguments to methods are evaluated once, when the method is created and that value is used for all calls. I don't know why that is, its definitely strange - any other language would evaluate the default args each time a function is called just like the regular args.

Then you would have the problem that something like mydefault=3, def f(default=mydefault):... would change it's evaluation if anyone ever defined mydefault=4. That would leave users unable to define any variable thats ever mentioned in default values of any function in any library they are using without causing very unexpected behaviors. What languages are you thinking of where default values set to a references are…

    scala> var x = 5
    x: Int = 5

    scala> def func(arg1: Int = (x+1)) = { println(arg1)}
    func: (arg1: Int)Unit

    scala> func()
    6

    scala> var x = 7
    x: Int = 7

    scala> func()
    6 //unchanged

    scala> var list = List(1,2,3)
    list: List[Int] = List(1, 2, 3)

    scala> def printList(arg1: List[Int] = lis) = { println(arg1) }
    printList: (arg1: List[Int])Unit

    scala> printList()
    List(1, 2, 3)

    scala> var list = List(4,5,6)
    list: List[Int] = List(4, 5, 6)

    scala> printList()
    List(1, 2, 3)