CODEWITHSUNDEEP
pythondecorator
tponthieux
tponthieux

Reputation: 1552

Is it possible to retrieve a value in a child class generated by a decorator defined in a base class?

I have a child class method I want to conditionally short circuit. What I'm trying to do is something like this except I want to put the validation logic into the base class.

class BaseClass(object):
    def getvalue(self):
        return True
    def validate(self):
        validated = self.getvalue()
        return validated

class ExtendedClass1(BaseClass):
    def do_some_work(self):
        validated = self.validate()
        if not validated:
            print "Not validated."
            return
        print "Things are validated if the method got this far.", validated

class ExtendedClass2(BaseClass):
    def do_some_work(self):
        validated = self.validate()
        if not validated:
            print "Not validated."
            return
        print "Things are validated if the method got this far.", validated

class ExtendedClass3(BaseClass):
    def do_some_work(self):
        print "This one doesn't require validation."

work1 = ExtendedClass1()
work1.do_some_work()

work2 = ExtendedClass2()
work2.do_some_work()

work3 = ExtendedClass3()
work3.do_some_work()

Following this example I was able to convert some of the repeated code into a decorator pattern.

class BaseClass(object):

    def validate(input_function):
        def wrapper(*args,**kwargs):
            validated = True
            if not validated:
                print "Not validated."
                return
            input_function(*args, **kwargs)
        return wrapper

    validate = staticmethod(validate)

class ExtendedClass1(BaseClass):
    @BaseClass.validate
    def do_some_work(self):
        print "Things are validated if the method got this far."

class ExtendedClass2(BaseClass):
    @BaseClass.validate
    def do_some_work(self):
        print "Things are validated if the method got this far."

class ExtendedClass3(BaseClass):
    def do_some_work(self):
        print "This one doesn't require validation."

work1 = ExtendedClass1()
work1.do_some_work()

work2 = ExtendedClass2()
work2.do_some_work()

work3 = ExtendedClass3()
work3.do_some_work()

However, I need to call a method of the base class in the decorator to do the validation work, and retrieve the value of (validated) in the child class. Following this example here, I modified the decorator in an attempt to let it call self.getvalue(). At this point it doesn't error out, but it doesn't work either because self.getvalue() doesn't return True. This is starting to seem like more trouble than it's worth, but now I'm curious as to whether or not it's possible.

class BaseClass(object):

    def getvalue(self):
        return True

    def validate(self):
        def wrap(input_function):
            def wrapper(*args,**kwargs):
                validated = self.getvalue()
                if not validated:
                    print "Not validated."
                    return
                input_function(*args, **kwargs)
            return wrapper
        return wrap

    validate = staticmethod(validate)

class ExtendedClass1(BaseClass):
    @BaseClass.validate
    def do_some_work(self):
        print "Things are validated if the method got this far."#, validated

class ExtendedClass2(BaseClass):
    @BaseClass.validate
    def do_some_work(self):
        print "Things are validated if the method got this far."#, validated

class ExtendedClass3(BaseClass):
    def do_some_work(self):
        print "This one doesn't require validation."

work1 = ExtendedClass1()
work1.do_some_work()

work2 = ExtendedClass2()
work2.do_some_work()

work3 = ExtendedClass3()
work3.do_some_work()

Is it possible to set an attribute with the decorator and then retrieve it later?

                ...
                self.validated = True
                if not self.validated:
                    print "Not validated."
                    return
                ...
print work1.validated
                ...

AttributeError: 'ExtendedClass1' object has no attribute 'validated'

Essentially, I want to turn this:

class ExtendedClass1(BaseClass):
    def do_some_work(self):
        validated = self.validate()
        if not validated:
            print "Not validated."
            return
        print "Things are validated if the method got this far.", validated

Into this:

class ExtendedClass1(BaseClass):
    @BaseClass.validate
    def do_some_work(self):
        print "Things are validated if the method got this far.", validated

Using the suggestion posted by Zaur Nasibov, this example satisfies my use case. I'm still interested to know if @validate can be implemented as a method instead of a stand alone function, but this gets the job done.

class BaseClass(object):
    def getvalue(self):
        return True

def validate(func):
    def wrapped(self, *args, **kwargs):
        validated = self.getvalue()
        self.validated = validated
        if not validated:
            print "Not validated."
            return
        func(self, *args, **kwargs)
    return wrapped

class ExtendedClass1(BaseClass):
    @validate
    def do_some_work(self,input):
        print "Things are validated if the method got this far.", self.validated, input

class ExtendedClass2(BaseClass):
    @validate
    def do_some_work(self):
        print "Things are validated if the method got this far.", self.validated

class ExtendedClass3(BaseClass):
    def do_some_work(self):
        print "This one doesn't require validation."#, self.validated

work1 = ExtendedClass1()
work1.do_some_work(input="some text")

work2 = ExtendedClass2()
work2.do_some_work()

work3 = ExtendedClass3()
work3.do_some_work()

Upvotes: 0

Views: 98

Answers (2)

Martijn Pieters
Martijn Pieters

Reputation: 1121844

How about only calling the decorated method if it has been validated? You can pass the return value of the validate method along if you like:

class BaseClass(object):
    def getvalue(self):
        return True

    def validate(input_function):
        def wrapper(self, *args, **kwargs):
            self.validated = self.getvalue()
            if not self.validated:
                print "Not validated."
                return
            input_function(self, validated=self.validated, *args, **kwargs)
        return wrapper

    validate = staticmethod(validate)

class ExtendedClass1(BaseClass):
    @BaseClass.validate
    def do_some_work(self, validated=None):
        print "Things are validated if the method got this far.", validated

class ExtendedClass2(BaseClass):
    @BaseClass.validate
    def do_some_work(self, validated=None):
        print "Things are validated if the method got this far.", validated

class ExtendedClass3(BaseClass):
    def do_some_work(self):
        print "This one doesn't require validation."

work1 = ExtendedClass1()
work1.do_some_work()

work2 = ExtendedClass2()
work2.do_some_work()

work3 = ExtendedClass3()
work3.do_some_work()

The key here is adding self to the wrapper function. What happens is that your decorated functions do not get bound to the instance (and become methods), but the function returned by the decorator (wrapper in the above example) get's bound instead. So this function will get the self (the instance) parameter passed in when called! It's important to remember that what a @decorator does is simply call decorator passing in the function you are decorating, and then replace the function you are decorating with whatever the decorator returned. In your example this is wrapper, and to the class, there is no difference between that and the original function before decorating.

In the above example, I declared self explicitly. If we hadn't, we could also have just taken it from args:

def validate(input_function):
    def wrapper(*args, **kwargs):
        print "args[0] is now the instance (conventionally called 'self')", args[0]
        self = args[0]
        self.validated = self.getvalue()
        if not self.validated:
            print "Not validated."
            return
        input_function(validated=self.validated, *args, **kwargs)

Also note that we pass an extra keyword argument to the wrapped method, named validated. This is entirely optional, you can just drop the validated=self.validated and validated=None parts from the example.

Upvotes: 1

Zaur Nasibov
Zaur Nasibov

Reputation: 22659

@tponthieux, what you can do is setting the attribute of the called function (method) and then retrieving it:

Simple example (updated):

def validate(func):    
    def wrapped(self, *args, **kwargs):
        self.valid = True
        func(self, *args, **kwargs)
    return wrapped

class TestClass(object):
    @validate
    def do_some_work(self):
        print "some work done"

tc = TestClass()
tc.do_some_work()
print tc.valid

Upvotes: 1

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