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Binding python variable to specific expression

I am developing a simple application that contains a Constants.py file containing all the configuration as it looks like

x = y

during program execution the y value changes, I want the xo value to update automatically as well, this can be specified as a binding, how can I achieve this

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4 answers


Python variable names indicate the value. x=y

tells Python that the variable name x

should point to the value it currently points to y

.

When you change y

, the variable name y

points to the new value, and the variable name x

still points to the old value.

You cannot achieve what you want with simple variable names.

I like KennyTM's proposal for defining x

as a function, as it makes it explicit that the value x

requires some code to run (lookup for the y value).

However, if you want to maintain a consistent syntax (making all constants available the same way), you can use a class with properties (attributes that 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


This is where you change the "constant" y:

const.y='bar'

And the "constant" x also changes:

print(const.x)
# bar

You can also change x

,

const.x='foo'

and y

also changes:

print(const.y)
# foo
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If you change the value (object), then all references to it will be updated:

>>> a = []
>>> b = a  # b refers to the same object a is refering right now
>>> a.append('foo')
>>> print b
['foo']

However, if you give a name to some other object, then the other names will refer to whatever they referred to before:



>>> a = 15
>>> print b
['foo']

How does python. Names are just references to objects. You can make a name reference to the same object referenced by another name, but you cannot assign a name reference to a different name. Attributing a name using the =

( a = 15

) operator changes the referenced value a

, so it cannot affect other names.

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if your config values ​​are inside a class, you can do something like this:

>>> class A(object):
...     a = 4
...     @property
...     def b(self):
...         return self.a
...

then every time you access b it will return the value of a.

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There is a simple solution you can make. Just define the property and set the value fget

you defined.

For instance:

a = 7

@property 
def b():
    return a

if you ask for b you get something like this <property object at 0x1150418>

, but if you do b.fget()

you get the value 7

Now try this:

a = 9
b.fget()  # this will give you 9. The current value of a

You don't need to have a class that way, otherwise I think you need to.

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