Using decorator on a variable in a function in Python

Question

I would like to use a decorator for validating data in Python. Normally in Python one uses a decorator on a function, but I would like to use a decorator on a variable, similar to Java:

public class Main {
   public static void main(String[] args) {
      @BoldWrapper
      @ItalicWrapper
      String str = "Hello World";
      // Display <b><i>Hello World</i></b>
   }
}

public @interface BoldWrapper {
    public void wrap() default "<b>" + str + "</b>";
}

public @interface ItalicWrapper {
    public void wrap() default "<i>" + str + "</i>";
}

So in Python I would have something like:

if __name__ == '__main__':
    @BoldWrapper
    @ItalicWrapper
    str = "Hello World";

@fechnert - you can decorate getters/setters which would, from the outside, appear as properties. — zwer
– zwer, Commented Jun 6, 2017 at 9:43
@zwer - which are functions in the first place ;) but yeah, thats also a possible way — fechnert
– fechnert, Commented Jun 6, 2017 at 9:44
Java annotations != Python decorators... see this discussion — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Jun 6, 2017 at 9:46

Azat Ibrakov · Accepted Answer · 2017-06-06 10:42:55Z

1

only classes and functions/methods definitions may be decorated (functions/methods since Python 2.4, classes since Python 2.6):

from docs about function definition

A function definition may be wrapped by one or more decorator expressions. Decorator expressions are evaluated when the function is defined, in the scope that contains the function definition.

and class definition

Classes can also be decorated: just like when decorating functions

so

@BoldWrapper
@ItalicWrapper
text = "Hello World"

will cause SynaxError.

There is no need for this feature, because we can just write function calls

text = BoldWrapper(ItalicWrapper("Hello World"))

P. S.

do not use names of built-ins like str for your objects,
semicolons are redundant in Python

@Petronella no, it is a syntax error. The decorator syntax is syntactic sugar for f = deco(f) after a function definition. It merely exists for the added benefit of readability - one often wants to see the decorators before reading the function body. Decorators in Python are not the same thing as annotations in Java.

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