python decorator to check for already called func with unique arguments

Question

I m writing python decorator to check if func was previously called with same arguments.

Below is proof of concept code

storage = list()
def f(*args, **kwargs):

    s = ''
    for i in range(len(args)):
        s += str(args[i])
    kv = ''
    for k, v in kwargs.items():
        kv += str(k) + str(v)

    main = s+kv
    if main not in storage:
        storage.append(main)
    else:
        print('this was called!')
    print('printing storage')
    print(storage)



if __name__ == '__main__':


    f(1, 2, 3, 4, 5, 6, 7, 8, x=10)
    f(1, 2, 3, 4, 5, 6, 7, 8, x=10, z=10)
    f(1, 2, 3, 4, 5, 6, 7, 8, x=10, z=10) #this combination of args, kwargs should be skipped by the function f

My actual decorator fails with this error msg:

"TypeError(‘can only concatenate tuple (not "dict") to tuple’)"

challenge website link

here i simply turn the list of args into str and **kwargs dict into string and concatenate them to create unique combination of args/kwargs called and store it in storage list

e.g. func(1,2,3,x=3,b=4) => 123x3b4

class Answer:
    def RepeatDecorator(self, func):
        self.storage = list()
        def wrapper(*args, **kwargs):
            s = ''
            for i in range(len(args)):
                s += str(args[i])
            kv = ''
            for k, v in kwargs.items():
                kv += str(k) + str(v)

            main = s+kv
            if main not in self.storage:
                self.storage.append(main)
                func(args, kwargs)
            else:
                print("func with this args was already called, do nothing")                                        
        return wrapper

e.g.

func1(1,x=2)
func1(1,x=3,b=4)
func2(1,x=2)
func2(1,x=3,b=4

all 4 should work and be stored, since func1 and func2 are different functions

Asked By: ERJAN

||

Source

Answer 1

Here are some issues I found with your code:

Different functions given to Answer.RepeatDecorator will share the same storage list, if they’re using the same Answer instance.
Pasting strings together is ambiguous. This will treat func(12, 3) the same as func(1, 23) and func(x=4) the same as func('x4').
Looking up strings in a list is fairly slow. In this case it’s unlikely to be a problem, but you’ll probably want to use a set if this kinda thing is needed in the future (as long as the arguments are hashable).
You’re calling func(args, kwargs) when you should call func(*args, **kwargs). It’s something I often overlook when writing decorators.

class Answer:
    def RepeatDecorator(self, func):
        # Use a local variable rather than an attribute on self
        storage = []
        def wrapper(*args, **kwargs):
            key = (args, kwargs)
            if key not in storage:
                storage.append(key)
                # It doesn't say you need to return the value func returns, but it can't hurt, right?
                return func(*args, *kwargs)
            print("func with this args was already called, do nothing")                                        
        return wrapper

For completeness sake, here’s how I would do it using a set:

class Answer:
    def RepeatDecorator(self, func):
        storage = set()
        def wrapper(*args, **kwargs):
            key = (args, tuple(kwargs.items()))
            if key not in storage:
                storage.add(key)
                return func(*args, *kwargs)
            print("func with this args was already called, do nothing")                                        
        return wrapper

Answered By: Jasmijn

python decorator to check for already called func with unique arguments

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