How to Reference Global Variables and Class Variables

How to reference global variables and class variables?

Global scope is scope that covers the entire program. Global scope is enjoyed by global variables, which are recognizable by their initial dollar-sign ($) character. They’re available everywhere and creating your own global variables can be tempting, especially for beginning programmers. But they’re not always a good idea.

$gvar = "I'm a global!"
class C
    def examine_global
        puts $gvar
    end
end

c = C.new
c.examine_global # I'm a global!

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Class variables begin with two at signs: @@var, for example. Despite their name, class variables aren’t class scoped. Rather, they’re class-hierarchy scoped. At its simplest, the idea behind a class variable is that it provides a storage mechanism that’s shared between a class and instances of that class, and that’s not visible to any other objects.

class Parent
    @@value = 100
end

class Child < Parent
    @@value = 200
end

class Parent
    puts @@value
end

What gets printed is 200. The Child class is a subclass of Parent, and that means Parent and Child share the same class variables—not different class variables with the same names, but the same actual variables. When you assign to @@value in Child, you’re setting the one and only @@value variable that’s shared throughout the hierarchy—
that is, by Parent and Child and any other descendant classes of either of them.

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And to give credit where its due - This explanation comes from "The Well Grounded Rubyist" by David A Black, one of the best resources to learn about Ruby.

How can I access global variable inside class in Python

By declaring it global inside the function that accesses it:

g_c = 0

class TestClass():
    def run(self):
        global g_c
        for i in range(10):
            g_c = 1
            print(g_c)

The Python documentation says this, about the global statement:

The global statement is a declaration which holds for the entire current code block.

Assign Global Variable Inside Class From Class Variable

__table__ is a class variabe, which means you can access it from an instance or from the class itself. You can update any instance value according to the value of backfill in the __init__ method:

class Model(Object):
    __table__ = 'table_name'
    def __init__(self, backfill = False):
        self.backfill = backfill
        if self.backfill:
            self.__table__ = 'table_name2'

Then to create an instance, just give the backfill parameter to the constructor, without any if/else statement:

print(Model.__table__)
# table_name
backfill = True
model = Model(backfill)
print(model.__table__)
# table_name2

But I don't see the point of using a class variable in your case. You can just define it in the __init__ method:

class Model(Object):
    def __init__(self, backfill = False):
        self.backfill = backfill
        self.__table__ = 'table_name'
        if self.backfill:
            self.__table__ = 'table_name2'

Global variable inside the class and local function variable

Use this->a.

this allows you to access the members and methods of an instance from within.

EDIT: this is an academical exercise, but a bad programming practice. Just use different names for classes, members and variables.

How to define global variable in class in python

If you just want to define your variable outside of the class you don't need to use the global keyword unless you plan on modifying it. If you just want to read the variable and not modify it you could just do something like.

ask = input("What type you are writing? (absolute value and phase angle or real and imaginary parts)")

class ComplexMethods:
    if ask == "real and imaginary parts":
        pass

if ask == "real and imaginary parts":
    firstcomplexreal = float(input("Enter real part of first complex number: "))
    firstcompleximaginary = float(input("Enter imaginary part of first complex number: "))
    secondcomplexreal = float(input("Enter real part of second complex number: "))
    secondcompleximaginary = float(input("Enter imaginary part of second complex number: "))
    complexnumbers = ComplexMethods(firstcomplexreal, firstcompleximaginary, secondcomplexreal,
                                    secondcompleximaginary)

Global variables in Java

To define Global Variable you can make use of static Keyword

public class Example {
    public static int a;
    public static int b;
}

now you can access a and b from anywhere
by calling

Example.a;

Example.b;

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