Assigning return value of a class method to a variable error

around this issue There's nothing that does this in a shorter way.
To be fair, it is an unusual pattern, and while not especially rare, would lead to confusion if there was an operator like:

@value .= some_method

I wish this helpful for you Your understanding is wrong. self is itself a parameter to that function definition, so there's no way it can be in scope at that point. It's only in scope within the function itself.
The answer is simply to default the argument to None, and then check for that inside the method:

def doSomething(self, a=None):
    if a is None:
        a = self.z
    self.z = 3
    self.b = a

like below fixes the issue This is called runtime polymorphism.
JVM decides at runtime which method to call based on the content of the object not the type of object.

class Animal {
  void whoAmI() {
    System.out.println("I am a generic Animal.");
  }
}
class Dog extends Animal {
  void whoAmI() {
    System.out.println("I am a Dog.");
  }
}
class Cow extends Animal {
  void whoAmI() {
    System.out.println("I am a Cow.");
  }
}
class Snake extends Animal {
  void whoAmI() {
    System.out.println("I am a Snake.");
  }
}

class RuntimePolymorphismDemo {

  public static void main(String[] args) {
    Animal ref1 = new Animal();
    Animal ref2 = new Dog();
    Animal ref3 = new Cow();
    Animal ref4 = new Snake();
    ref1.whoAmI();
    ref2.whoAmI();
    ref3.whoAmI();
    ref4.whoAmI();
  }
}
The output is

I am a generic Animal.
I am a Dog.
I am a Cow.
I am a Snake.

like below fixes the issue This is because File.ReadAllLines() returns an array of strings. You can read more about it here.
Instead, try it like this:

Private Sub dlsuc()
    Dim file_ As String = My.Computer.FileSystem.SpecialDirectories.Temp & "\v.txt"
    Dim file As String() = IO.File.ReadAllLines(file_)
End Sub

may help you . Basically, in your example and the linked example of the iterator, assigning the return value is unnecessary.
In your example it is because there is no return value. In other words, the function returns void which will cause $var to be null. In general, if a function contains no return statement at all or only a return; (note that there is no variable being returned), it is useless to store the result in a variable. In the iterator example it is unnecessary, because the value isn't used. But the $current variable does still contain the actual value of the current iteration - it is not null (except when the array contains null, obviously). Actually calling $this->current() wouldn't be necessary at all in the iterator implementation - it looks like it is done solely for the purpose of getting the nice output of echo "current: $var\n"; when checking for the validity.

function foo() {
    echo 'bar';
}

$var = 'foo';
$var(); // outputs: bar