Python decorator for accessing class variable

Question

I have a decorator to control time limit, if the function execution exceeds limit, an error is raised.

def timeout(seconds=10):
    def decorator(func):
        # a timeout decorator
    return decorator

And I want to build a class, using the constructor to pass the time limit into the class.

def myClass:
    def __init__(self,time_limit):
        self.time_limit = time_limit

    @timeout(self.time_limit)
    def do_something(self):
        #do something

But this does not work.

File "XX.py", line YY, in myClass
    @timeout(self.tlimit)
NameError: name 'self' is not defined

What's the correct way to implement this?

Answer 1

self.time_limit is only available when a method in an instance of your class is called.

The decorator statement, prefixing the methods, on the other hand is run when the class body is parsed.

However, the inner part of your decorator, if it will always be applied to methods, will get self as its first parameter - and there you can simply make use of any instance attribute:

def timeout(**decorator_parms):
    def decorator(func):
        def wrapper(self, *args, **kwargs):
             time_limit = self.time_limit
             now = time.time()
             result = func(self, *args, **kwargs)
             # code to check timeout
             ..
             return result
        return wrapper
    return decorator

If your decorator is expected to work with other time limits than always self.limit you could always pass a string or other constant object, and check it inside the innermost decorator with a simple if statement. In case the timeout is a certain string or object, you use the instance attribute, otherwise you use the passed in value;