I would like to have a variable which is updated every time it is accessed relative to other variables it is composed of like:
string1 = "a"
string2 = "b"
string3 = string1 + string2
print(string3)
gives ab
string1 = "c"
print(string3)
gives cb
Is there a simple way in Python to achieve that?
I am unaware if there's a better way to do this, or if you know this method, but asking for something better, but you can try this:
string1 = "a"
string2 = "b"
string3 = lambda:string1 + string2
print(string3())
>>> 'ab'
string1 = "c"
print(string3())
>>> 'cb'
a = b will never mutate what was in a before; it will bind the name a to a new value. Nothing else will be affected by this, since anything can only reference the value, not the name. So what you need is a mutable value; anything that holds a reference to that mutable object will see any mutations on it:
class Foo:
def __init__(self, value):
self.value = value
def __str__(self):
return self.value
class Bar:
def __init__(self, *values):
self.values = values
def __str__(self):
return ''.join(map(str, self.values))
string1 = Foo('a')
string2 = Foo('b')
string3 = Bar(string1, string2)
print(string3)
string1.value = 'c'
print(string3)
Depending on what exactly you need this for, you might also consider using custom class with a property - all values would be then contained in a single class, not as many objects (in case of many objects, string1=something mistyped instead of string1.value=something breaks your relation because it's not longer the same object).
You can access the property just like a normal member of the class (without parentheses) but its result may be dependent on other values (because behind the scenes, it's still a function).
class Test:
def __init__(self, string1, string2):
self.string1 = string1
self.string2 = string2
@property
def string3(self):
return self.string1 + self.string2
Example of usage:
>>> test = Test("a","b")
>>> print(test.string1, test.string2, test.string3)
a b ab
>>> test.string1 = "c"
>>> print(test.string1, test.string2, test.string3)
c b cb
()call, which makes it clear when exactly data changed and where the changed data came from.