instance-variables

I am a little bit confused about the following piece of code: public class Test{ int x = giveH(); int h = 29; public int giveH(){ return h; } public static void main(String args[]) { Test t = new Test(); System.out.print(t.x + " "); System.out.print(t.h); } } The output here is 0 29 , but I thought that this has to be a compiler error, because the variable h should have not been initialized when it comes to the method giveH() . So, does the compilation go through the lines from top to bottom? Why is this working? Why is the value of x 0 and not 29? The default value of int is 0 (see here ).

How would you I came up with this interesting (at least to me) example. import numpy as np class Something(object): a = np.random.randint(low=0, high=10) def do(self): self.a += 1 print(self.a) if __name__ == '__main__': something = Something() print(something.__str__()) something.do() something2 = Something() print(something2.__str__()) something2.do() something3 = Something() print(something3.__str__()) something3.do() The above prints the following in the console: $ python test.py <__main__.Something object at 0x7f03a80e0518> 1 <__main__.Something object at 0x7f03a80cfcc0> 1 <__main__

Python provides us many possibilities on instance/class attribute, for example: class A(object): def __init__(self): self.foo = "hello" a = A() There are many ways to access/change the value of self.foo : direct access a.foo inner dict a.__dict__['foo'] get and set a.__get__ and a.__set__ ,of course there two are pre-defined methods. getattribute a.__getattribute__ __getattr__ and __setattr__ maybe more. While reading source code, I always get lost of what's their ultimate access order? When I use a.foo , how do I know which method/attribute will get called actually? bar = a.foo ... invokes a.

Or can they be declared otherwise? The code below does not work: class BinaryNode(): self.parent = None self.left_child = None Do they need to be declared in __init__ ? They do not have to be declared in __init__ , but in order to set an instance variable using self , there needs to be a reference to self , and the place you are defining the variables does not. However, class BinaryNode(): parent = None left_child = None def run(self): self.parent = "Foo" print self.parent print self.left_child The output will be Foo None To answer your question in the comment, yes. You can, in my example say:

Unsure of the Python convention for type hinting instance variables - I've been doing them within the __init__ constructor arguments like seen here: class LoggedVar(Generic[T]): def __init__(self, value: T, name: str, logger: Logger) -> None: self.name = name self.logger = logger self.value = value` link to above code snippet: https://docs.python.org/3/library/typing.html#user-defined-generic-types But I also see the PEP conventions of annotating instance variables as such(snippet below) and then also doing type hinting within the __init__ arguments: class BasicStarship: captain: str = 'Picard

I'm implementing an object that is almost identical to a set, but requires an extra instance variable, so I am subclassing the built-in set object. What is the best way to make sure that the value of this variable is copied when one of my objects is copied? Using the old sets module, the following code worked perfectly: import sets class Fooset(sets.Set): def __init__(self, s = []): sets.Set.__init__(self, s) if isinstance(s, Fooset): self.foo = s.foo else: self.foo = 'default' f = Fooset([1,2,4]) f.foo = 'bar' assert( (f | f).foo == 'bar') but this does not work using the built-in set module.

This question already has an answer here: How do servlets work? Instantiation, sessions, shared variables and multithreading 8 answers When I read Head First Servlet and JSP , they say that instance variable is non-thread safe. I don't understand this statement so much. For example: I have a servlet which name is ActionServlet.java . Each time, each user's request is sent to server, container will create a new thread and create new ActionServlet instance. ActionServlet may be has a structure: public class ActionServlet extends HttpServlet { // example of instance variable Instance variable;