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Decorators

Remember function transformations, where a (higher-order) function takes a function and returns a function with new behavior? Python decorators offer a kind of syntactic sugar around that. ("Syntactic sugar" just means "a more convenient syntax.")

Example:

from collections.abc import Callable


def vowel_counter(func_to_decorate: Callable[[str], None]) -> Callable[[str], None]:
    vowel_count: int = 0

    def wrapper(doc: str) -> None:
        nonlocal vowel_count
        vowels: str = "aeiou"
        for char in doc:
            if char.lower() in vowels:
                vowel_count += 1
        print(f"Vowel count: {vowel_count}")
        func_to_decorate(doc)

    return wrapper


@vowel_counter
def process_doc(doc: str) -> None:
    print(f"Document: {doc}")


process_doc("What")
# Vowel count: 1
# Document: What

process_doc("A wonderful")
# Vowel count: 5
# Document: A wonderful

process_doc("world")
# Vowel count: 6
# Document: world

The @vowel_counter line is "decorating" the process_doc function with the vowel_counter function. vowel_counter is called once when process_doc is defined with the @ syntax, but the wrapper function that it returns is called every time process_doc is called. That's why vowel_count is preserved and printed after each time.

It's Just Syntactic Sugar

Python decorators are just another (sometimes simpler) way of writing a higher-order function. These two pieces of code are identical:

With Decorator

@vowel_counter
def process_doc(doc: str) -> None:
    print(f"Document: {doc}")


process_doc("Something wicked this way comes")

Without Decorator

def process_doc(doc: str) -> None:
    print(f"Document: {doc}")


process_doc = vowel_counter(process_doc)
process_doc("Something wicked this way comes")

Assignment

The provided file_type_aggregator function is intended to decorate other functions. It assumes that the function it decorates has exactly 2 positional arguments.

Create a process_doc function that is decorated by file_type_aggregator. It should return the following string:

f"Processing doc: '{doc}'. File Type: {file_type}"

Where doc and file_type are its positional arguments. (See the line where result is assigned to func_to_decorate(doc, file_type).)