Question

I am developing a simple application which hava a file Constants.py containing all configuration, it is like this

x = y

during execution of program , the value of y changes , I want value of x o get updated too , automatically, this can be reffered as binding, how can I achieve this

Answer 1

In Python variable names point at values. x=y tells Python that the variable name x should point at the value that y is currently pointing at.

When you change y, then the variable name y points at a new value, while the variable name x still points at the old value.

You can not achieve what you want with plain variable names.

I like KennyTM’s suggestion to define x as a function since it makes explicit that the value of x requires running some code (the lookup of the value of y).

However, if you want to maintain a uniform syntax (making all the constants accessible in the same way), then you could use a class with properties (attributes which call getter and setter functions):

Constants.py:

class BunchOConstants(object):
    def __init__(self, **kwds):
        self.__dict__.update(kwds)
    @property
    def x(self):
        return self.y
    @x.setter
    def x(self,val):
        self.y=val
const=BunchOConstants(y=10,z='foo')

Your script.py:

import Constants

const=Constants.const
print(const.y)
# 10
print(const.x)
# 10

Here you change the “constant” y:

const.y='bar'

And the “constant” x is changed too:

print(const.x)
# bar

You can change x also,

const.x='foo'

and y too gets changed:

print(const.y)
# foo