I'm trying to add type hints to a Python function that flattens a list of lists. Here's the function I've been using:
```python
def flatten(list_of_lists):
return list(itertools.chain.from_iterable(list_of_lists))
```
This method is pretty common for flattening lists. I even tried another variation of the function:
```python
def flatten(list_of_lists):
return list(itertools.chain(*list_of_lists))
```
However, I'm having issues with type hinting using pyright, and it doesn't seem to accept my attempts. What would be the correct way to add type hints to this function?
2 Answers
That's interesting! Just a heads up, if you use a type variable such as `T`, you could declare it like this: `T = TypeVar("T")` and then type hint your flatten function as follows:
```python
def flatten(list_of_lists: Iterable[T]) -> Iterable[T]:
return list(chain.from_iterable(list_of_lists))
``` Just keep in mind to be as general as possible with input types and more specific with output types – consider returning `list[T]` instead of `Iterable[T]`.
You might want to use a type hint like `def flatten[T](lol: Iterable[Iterable[T]]) -> list[T]`. This approach should work well for what you're trying to accomplish.
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