deco.py

from typing import Optional

def maybe(func):
    def fn(*args, **kwargs):
        try:
            ret = func(*args, **kwargs)
            if ret:
                return [ret]
            else:
                return[]
        except Exception:
            return []
    return fn

@maybe
def findUser(code: str) -> Optional[dict]:
    users = [
        {"id": 1, "code": 2012, "name": "yuzu"},
        {"id": 2, "code": 3308, "name": "yui"},
    ]
    for user in users:
        if user["code"] == code:
            return user
    return None

@maybe
def findItem(userId: int) -> Optional[dict]:
    items = [
        {"userId": 1, "name": "keyboard"}
    ]
    for item in items:
        if item["userId"] == userId:
            return item
    return None

@maybe
def parseInt(input: str) -> Optional[int]:
    return int(input)


if __name__ == '__main__':

    kbd = [item["name"]
        for code in parseInt("2012")
        for yuzu in findUser(code)
        for item in findItem(yuzu["id"])]

    print(kbd)  # => ['keyboard']
    
    missing = [item["name"]
        for code in parseInt("3308")
        for yui in findUser(code)
        for item in findItem(yui["id"])]
    print(missing)  # => []

    what = [item["name"]
        for code in parseInt("wxyz")
        for who in findUser(code)
        for item in findItem(who["id"])]
    print(what)  # => []

gen.py

class MaybeIterator:
    def __init__(self, value):
        self.value = value
        self.index = -1
    
    def __next__(self):
        self.index += 1
        if self.index == 0 and self.value:
            return self.value
        else:
            raise StopIteration

class Maybe:
    def __init__(self, value):
        self.value = value
    
    def __iter__(self):
        return MaybeIterator(self.value)
    
    def getOrElse(self, other):
        if self.value:
            return self.value
        else:
            return other

def findUser(code: str) -> Maybe:
    users = [
        {"id": 1, "code": 2012, "name": "yuzu"},
        {"id": 2, "code": 3308, "name": "yui"},
    ]
    for user in users:
        if user["code"] == code:
            return Maybe(user)
    return Maybe(None)

def findItem(userId: int) -> Maybe:
    items = [
        {"userId": 1, "name": "keyboard"}
    ]
    for item in items:
        if item["userId"] == userId:
            return Maybe(item)
    return Maybe(None)

def parseInt(input: str) -> Maybe:
    try:
        return Maybe(int(input))
    except ValueError:
        return Maybe(None)


if __name__ == '__main__':

    kbd = [item["name"]
        for code in parseInt("2012")
        for yuzu in findUser(code)
        for item in findItem(yuzu["id"])]

    print(kbd)  # => ['keyboard']
    
    missing = [item["name"]
        for code in parseInt("3308")
        for yui in findUser(code)
        for item in findItem(yui["id"])]
    print(missing)  # => []

    what = [item["name"]
        for code in parseInt("wxyz")
        for who in findUser(code)
        for item in findItem(who["id"])]
    print(what)