
1. Use Type Hints

   Add type hints for all methods and the product_ids attribute to improve code clarity and compatibility with static code analyzers like mypy. This will also enhance the quality of auto-generated documentation.


from typing import List, Iterator

   Update the product_ids attribute and methods to include type hints:


class ProductIdList:
    """
    A list of ProductId objects.

    Attributes:
        product_ids (List[ProductId]): A list of ProductId objects.
    """
    def __init__(self, product_ids: List[ProductId]) -> None:
        ...

    def __iter__(self) -> Iterator[ProductId]:
        ...

    def __len__(self) -> int:
        ...

    def __getitem__(self, index: int) -> ProductId:
        ...

    def __setitem__(self, index: int, value: ProductId) -> None:
        ...

    def __delitem__(self, index: int) -> None:
        ...

    def append(self, product_id: ProductId) -> None:
        ...

    def insert(self, index: int, product_id: ProductId) -> None:
        ...

    def remove(self, product_id: ProductId) -> None:
        ...


2. Use collections.abc.MutableSequence

   Instead of manually implementing multiple magic methods (__getitem__, __setitem__, etc.), subclass collections.abc.MutableSequence. This will simplify the code and make it behave like a Python list while still allowing you to enforce custom logic.


from collections.abc import MutableSequence

class ProductIdList(MutableSequence):
    """
    A list-like collection of ProductId objects.
    """
    def __init__(self, product_ids: List[ProductId]) -> None:
        if not isinstance(product_ids, list) or not all(isinstance(product_id, ProductId) for product_id in product_ids):
            raise ValueError("product_ids must be a list of ProductId")
        if len(set(product_ids)) != len(product_ids):
            raise DuplicateIdError("Duplicate product_ids are not allowed")
        self._product_ids = product_ids  # Use a private attribute

    def __getitem__(self, index: int) -> ProductId:
        return self._product_ids[index]

    def __setitem__(self, index: int, value: ProductId) -> None:
        if not isinstance(value, ProductId):
            raise ValueError("product_id must be an instance of ProductId")
        self._product_ids[index] = value

    def __delitem__(self, index: int) -> None:
        del self._product_ids[index]

    def __len__(self) -> int:
        return len(self._product_ids)

    def insert(self, index: int, product_id: ProductId) -> None:
        if not isinstance(product_id, ProductId):
            raise ValueError("product_id must be an instance of ProductId")
        if product_id in self._product_ids:
            raise DuplicateIdError("Duplicate product_ids are not allowed")
        self._product_ids.insert(index, product_id)

   Now, you can remove the append and remove methods since MutableSequence provides default implementations that call insert and __delitem__, respectively.


5. Define __str__ or __repr__

   Consider adding __str__ or __repr__ methods to enhance debugging and logging. For example:


def __repr__(self) -> str:
    return f"ProductIdList({self._product_ids!r})"

def __str__(self) -> str:
    return f"ProductIdList with {len(self)} product_ids"


6. Use @staticmethod or @classmethod for Validation

   Instead of repeating validation logic in multiple methods (__init__, insert, __setitem__, etc.), factor it out into a reusable static method. This reduces code duplication.


@staticmethod
def _validate_product_id(product_id: ProductId) -> None:
    if not isinstance(product_id, ProductId):
        raise ValueError("product_id must be an instance of ProductId")

   Then use it in other methods:


def __setitem__(self, index: int, value: ProductId) -> None:
    self._validate_product_id(value)
    self._product_ids[index] = value


7. Consider Using dataclasses

   While your current class has a bit more logic than a typical dataclass, you could still use dataclasses for boilerplate reduction if you want to simplify initialization.


from dataclasses import dataclass, field

@dataclass
class ProductIdList:
    _product_ids: List[ProductId] = field(default_factory=list)

    def __post_init__(self):
        if len(set(self._product_ids)) != len(self._product_ids):
            raise DuplicateIdError("Duplicate product_ids are not allowed")

   You would need to adapt the rest of the class to use the dataclass-generated features.



8. Add Unit Tests

   Ensure your module has comprehensive unit tests to validate all edge cases. A good set of tests would cover:


Normal usage (adding, removing, and accessing ProductId objects)
Edge cases (empty lists, large lists, invalid types, duplicate IDs)
Exception raising for invalid inputs


9. Document Exceptions in Type Hints

   Use Python 3.12's typing enhancements to specify exceptions raised by methods. For example:


from typing import Annotated

def append(self, product_id: ProductId) -> Annotated[None, ValueError, DuplicateIdError]:
    ...

   This makes it clear to users of the API which exceptions to handle.



10. Use Python 3.12 Features (e.g., Self)

   Python 3.11 introduced typing.Self to represent the type of the current class. You can use it in methods that return the same type as the class:


from typing import Self

def copy(self) -> Self:
    return ProductIdList(self._product_ids[:])


Final Code with Improvements

Here’s a revised version of your class with the suggested improvements:

```python
from collections.abc import MutableSequence
from typing import List, Iterator
from functools import cached_property
from pact_methodology.exceptions import DuplicateIdError
from pact_methodology.urn import ProductId


class ProductIdList(MutableSequence):
    """
    A list-like collection of ProductId objects.

    Attributes:
        product_ids (List[ProductId]): A list of ProductId objects.
    """

    def __init__(self, product_ids: List[ProductId]) -> None:
        if not isinstance(product_ids, list) or not all(isinstance(product_id, ProductId) for product_id in product_ids):
            raise ValueError("product_ids must be a list of ProductId")
        if len(set(product_ids)) != len(product_ids):
            raise DuplicateIdError("Duplicate product_ids are not allowed")
        self._product_ids = product_ids

    def __getitem__(self, index: int) -> ProductId:
        return self._product_ids[index]

    def __setitem__(self, index: int, value: ProductId) -> None:
        self._validate_product_id(value)
        self._product_ids[index] = value

    def __delitem__(self, index: int) -> None:
        del self._product_ids[index]

    def __len__(self) -> int:
        return len(self._product_ids)

    def insert(self, index: int, product_id: ProductId) -> None:
        self._validate_product_id(product_id)
        if product_id in self._product_ids:
            raise DuplicateIdError("Duplicate product_ids are not allowed")
        self._product_ids.insert(index, product_id)

    @staticmethod
    def _validate_product_id(product_id: ProductId) -> None:
        if not isinstance(product_id, ProductId):
            raise ValueError("product_id must be an instance of ProductId")

    def __repr__(self) -> str:
        return f"ProductIdList({self._product_ids!
```