semvec/pypi-packages huggingface dataset:
arketip 0.5
automatic typeguards
License: MIT
Requires Python: >=3.10

# arketip

python3.10 introduced user-defined TypeGuards. Finally giving users the ability to actually code type narrowing functions themselves and avoid shortfalls of mypy. However, for most builtin basic types automatic TypeGuards are still missing. Writing these TypeGuards yourself for each of your types from scratch can feel redundant and frustrating. arketip is an effort to generate these TypeGuards automatically based solely on the type annotations.

## Example 1: TypeGuards

Here's an example of how you can use arketip to generate TypeGuards for a TypedDict. Let's suppose you have a TypedDict like this:

```python
class Person(TypedDict):
    uid: int
    name: str
```

You might already have observed that you can not use `isinstance` to check whether a variable is of that type.

```python
isinstance({"uid": 5, "name": "jeeves"}, Person)
# TypeError: TypedDict does not support instance and class checks
```

So how can you do type narrowing in that case? Well, you need to write a TypeGuard yourself.

```python
def is_person(data: Any) -> TypeGuard[Person]:
    return (isinstance(data, dict)
        and set(data.keys()) == {"uid", "name"}
        and isinstance(data["uid"], int)
        and isinstance(data["name"], str)
    )

is_person({"uid": 5, "name": "jeeves"}) # True
is_person({"name": "jeeves"}) # False
is_person({"uid": 5.0, "name": "jeeves"}) # False
is_person({"uid": 5, "name": b"jeeves"}) # False
```

As you can see this works perfectly, but these TypeGuards really feel like you are just duplicating the information that is already contained in the type, just to make mypy happy. Well, it's because it is actually duplicated. And even worse, for more complex nested types this gets out of hand quickly. arketip's `is_type` uses just the information from the type to automatically generate such TypeGuards for you.

```python
is_type({"uid": 5, "name": "jeeves"}, Person) # True
is_type({"name": "jeeves"}, Person) # False
is_type({"uid": 5.0, "name": "jeeves"}, Person) # False
is_type({"uid": 5, "name": b"jeeves"}, Person) # False
```

That's it, it's literally the same function, but automatically generated for you by arketip.

## Example 2: casting

Another use-case is casting, maybe you know that only one type is valid in a certain scenario. Let's say you again have the same TypedDict:

```python
class Person(TypedDict):
    uid: int
    name: str
```

with arketip, this is a breeze:

```python
jeeves = cast_to_type({"uid": 5, "name": "jeeves"}, Person)
```