一个新程序员关于 python 类和变量的基本问题

Question

I have a simple question about variable values being updated, please. I ran the following code:

class Number:
    def __init__(self,val):
        self.val=val

    def addthistoobj(self,obj):
        obj.val=self.val+obj.val

    def addthistovar(self,var):
        var=self.val+var

x=Number(7)
print("value of x is",x.val)

y=Number(10)
print("value  of y (obj) is",y.val)
x.addthistoobj(y)
print("value  of y (obj) after adding is",y.val)

p=10
print("value  of p (var) is",p)
x.addthistovar(p)
print("value  of p (var) after adding is",p)

The output I got was:

"

value of x is 7

value of y (obj) is 10

value of y(obj) after adding is 17

value of p (var) is 10

value of p (var) after adding is 10

"

The value of the object gets updated but the value of the variable does not.

Why is this the case?

Thank you for your time!

Answer 1

When you do x.addthistovar(p) , you are adding the value of x to p and creating a new object called var, but it "dies off" as the scope of the addthistovar function closes.

If you wanted to have p == 17 after that, you would need a return statement.

class Number:
    ...
    def addthistovar(self,var):
        return self.val + var
    ...

p = x.addthistovar(p)

Answer 2

Change your function to return the sum. As the value you are passing is not related to that object, thus can't be saved unless you define a field for it. You can save the result in a variable result and use it to your print statment.

   class Number:
        def __init__(self,val):
            self.val=val

    def addthistoobj(self,obj):
        obj.val=self.val+obj.val

    def addthistovar(self,var):
        return self.val+var

x=Number(7)
print("value of x is",x.val)

y=Number(10)
print("value  of y (obj) is",y.val)
x.addthistoobj(y)
print("value  of y (obj) after adding is",y.val)

p=10
print("value  of p (var) is",p)
result = x.addthistovar(p)
print("value  of p (var) after adding is", result)

Answer 3

This is because p never gets modified. Let's look at a more simple example:

def add_to(x, y):
   x = x + y

a, b = 1, 2

add_to(a, b)

This might lead you to believe that x will be re-assigned during the call to add_to . That's not the case. x in the scope of add_to becomes a different object, because the + operator for ints returns a new int. It also gets garbage collected (disappears) at the end of the scope of add_to . To reassign a , you would first need to return the new value, and re-assign it:

def add_to(x, y):
    return x + y # this returns a new integer created by x + y

a, b = 1, 2

a = add_to(a, b) # this reassigns the value 3 to a

Now for your class, you'd need to do the same thing:

class Number:
    def __init__(self,val):
        self.val=val

    def addthistoobj(self,obj):
        obj.val=self.val+obj.val

    def addthistovar(self,var):
        return self.val+var

p = 10
x = Number(7)

p = x.addthistovar(p)

p
17