如何在 Python 中更新变量中的值？

Question

I'm hoping to make a variable that changes when a value "in it" changes. That may sound confusing.

I'm new to Python, and I'm hoping to use this in future projects.

Here's an explanation. Say I have the variable foo and I want it to always be equal to bar plus three.

If I do

bar = 6
foo = bar+3

Then foo is equal to 9. But if I then do

bar = 5

Then foo is still 9. I'd like foo to be equal to 8, without executing

foo = bar+3

again. Is there anything I can do to make that happen?

Thanks.

EDIT: Thanks for the answers! I was already aware about how variables work. I guess using functions with return is the only way to do it.

Answer 1

foo can be defined like this:

foo = lambda: bar + 3

And can be used like this:

print(foo())

As you can see, foo is no longer a variable. foo is a function. foo can't be a variable because a variable doesn't suddenly change its value just because some other variable's value changed.

Answer 2

When you type

foo = bar + 3

It does not mean " foo is equal to three more than the value of bar ", it means " foo is equal to three more than the value of bar RIGHT NOW ."

If you need to do the former, you'll need to do some trickery.

class DelayedAdditionContext(object):
    def __init__(self, bar=0):
        self.bar = bar

    @property
    def foo(self):
        return self.bar + 3

context = DelayedAdditionContext()
context.bar = 5
context.foo  # 8
context.bar = 8
context.foo  # 11

but really this is just making a function under the hood.

def calculate_foo(bar):
    return bar + 3

bar = 5
foo = calculate_foo(bar)  # 8

bar = 8
foo = calculate_foo(bar)  # 11

Answer 3

Define a function to do this:

bar = 6
foo = bar + 3
# bar = 6, foo = 9

def set_bar(v):
    global bar, foo
    bar = v
    foo = bar + 3

set_bar(5)
# bar = 5, foo = 8

There is no general way to monitor variables in real time as I know.

Please see this post (object-oriented programmatically).

EDIT

---

To achieve this, you need to define your own type (that extended).

Answer 4

I think you can use an object-oriented method to define Bar and Foo in the same class. If you want to get the value of foo, call the method in the class to perform foo = bar+3. I don't know if I can help you. I am a rookie.