How To Use A Function Outside A Class As A Property Inside A Class?

I'm having some problems. How we can define a function outside of a function that can be used in a class property? Also, how we can insert the self parameter into the function sign

Solution 1:

It's not really clear what you want your out-of-class function to do. There are a bunch of possibilities, but you may not know the terminology yet to describe it to us.

Here's the three I think are most likely:

1. You may want your function to be a decorator. That means you can apply it to a method with @decorator syntax to other functions, including methods in a class.

   For this to work, your function needs to be written to accept a function object as its only argument. Whatever it returns is what will replace the function or method it was being called on, so usually you want to return a callable, but you could instead return a descriptor like property does. Try something like this:

   defdecorator(func):
       defwrapper(self, *args, **kwargs):
           print("in the wrapper")
           result = func(self, *args, **kwargs)
           print("wrapper is done")
           return result
       return wrapper
   
   classFoo:
       @decoratordeffoo(self, x):
           print("in foo(), x is", x)
   
   f = Foo()
   f.foo(1) # prints three messages

   When you call the foo method, you're actually going to be calling the wrapper method that the decorator returned after it was applied to the original method (func). Because of how we wrote the wrapper, it will call func so the original method prints out its message too.

2. You may want to use property (a descriptor type) to call your out-of-class function. This is a less common way of using property than applying it as a decorator on a method, but it's not impossible. You could even have two different functions, one to be called when requesting the attribute, the other than will be called when setting it (but I'll demonstrate with just the getter):

   defgetter(obj):
       print("in the getter")
       return1classFoo2:
       foo = property(getter)
   
   f2 = Foo2()
   print(f2.foo) # prints a message from the getter function first, then prints 1

   Note that you can't use @decorator syntax when building a property this way. That is only legal syntax immediately before a function definition, and we're not defining any functions that way inside our class.

3. You may just want to copy a function defined outside of the class into it, without any decorator or property nonsense. This is the easiest one to do, it's just a simple assignment:

   deffunc(self, x):
       print("x is", x)
   
   classFoo3:
       method = func  # just assign the global to a name in the class body
       func = func    # you can even use the same name if you don't mind confusing people
   
   f3 = Foo3()
   f3.method(1)
   f3.func(2)
   

Solution 2:

If you want to create a property that uses a function defined outside your class, it would be something like this:

defmyfunc(self):
    returnself._p

classFoo:def__init__(self, p):
        self._p = p
    p = property(myfunc)

f = Foo("Alpha")
f.p # gives "Alpha"

property accepts a function as its (first) argument. The function should have self as a parameter, and should return the value that you want the property to evaluate to.