Can Instance Attributes be Passed as Arguments to Class Method Decorators in Python?

Passing Class Fields to Class Method Decorators

In Python, class methods can be decorated using decorators that take additional arguments. However, questions arise when attempting to pass instance attributes as arguments to these decorators.

Consider the following example:

<code class="python">class Client(object):
    def __init__(self, url):
        self.url = url

    @check_authorization("some_attr", self.url)
    def get(self):
        do_work()</code>

The intent is to pass the self.url attribute to the check_authorization decorator. However, Python raises an error indicating that self does not exist in the class definition scope.

Solution

To resolve this issue, one can defer the attribute check to runtime by intercepting the method arguments in the decorator. The first argument to the wrapper function will be the instance, allowing access to instance attributes.

<code class="python">def check_authorization(f):
    def wrapper(*args):
        print(args[0].url)
        return f(*args)
    return wrapper

class Client(object):
    def __init__(self, url):
        self.url = url

    @check_authorization
    def get(self):
        print('get')

Client('http://www.google.com').get()
# Output: http://www.google.com
#          get</code>

Alternative Approach

Alternatively, one can dynamically access attributes by passing the attribute name as a string to the decorator. This eliminates the need to hardcode specific attribute names.

<code class="python">def check_authorization(attribute):
    def _check_authorization(f):
        def wrapper(self, *args):
            print(getattr(self, attribute))
            return f(self, *args)
        return wrapper
    return _check_authorization</code>

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