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Python Sort Hints – *args and **kwargs

Python Sort Hints – *args and **kwargs

2023-08-27 08:11:18

A typographer’s governor (apparently).

After I began writing kind hints, I used to be just a little confused about what to do with Python’s variable argument operators, * and ** (usually known as *args and **kwargs). Right here’s what I found out.

Recall that the * operator captures variable positional arguments in a tuple, and ** captures variable key phrase arguments in a dict. For instance, take this perform:

def variable(*args, **kwargs):
    ...

Within the perform physique, args might be a tuple, and kwargs a dict with string keys.

When including kind hints, it appears pure to attempt declare the total varieties of args and kwargs. If we wished all our values to be ints, we’d attempt:

def variable(*args: tuple[int, ...], **kwargs: dict[str, int]) -> None:
    ...

(The ... within the tuple definition makes it a tuple of any size.)

However that is incorrect. We are able to examine by including a name:

variable(1, 2, 3, a=4, b=5, c=6)

Operating Mypy on the file, it finds an issue with each argument(!):

$ mypy instance.py
instance.py:5: error: Argument 1 to "variable" has incompatible kind "int"; anticipated "Tuple[int, ...]"
instance.py:5: error: Argument 2 to "variable" has incompatible kind "int"; anticipated "Tuple[int, ...]"
instance.py:5: error: Argument 3 to "variable" has incompatible kind "int"; anticipated "Tuple[int, ...]"
instance.py:5: error: Argument "a" to "variable" has incompatible kind "int"; anticipated "Dict[str, int]"
instance.py:5: error: Argument "b" to "variable" has incompatible kind "int"; anticipated "Dict[str, int]"
instance.py:5: error: Argument "c" to "variable" has incompatible kind "int"; anticipated "Dict[str, int]"
Discovered 6 errors in 1 file (checked 1 supply file)

Uh oh! What’s the precise method then?

* all the time binds to a tuple, and ** all the time binds to a dict with string keys. Due to this restriction, kind hints solely want you to outline the varieties of the contained arguments. The kind checker mechanically provides the tuple[_, ...] and dict[str, _] container varieties.

The Python Enhancement Proposal (PEP) that launched kind hints, PEP 484, specified this rule:

Arbitrary argument lists can as effectively be kind annotated, in order that the definition:

def foo(*args: str, **kwds: int): ...

is appropriate… Within the physique of perform foo, the kind of variable args is deduced as Tuple[str, ...] and the kind of variable kwds is Dict[str, int].

So, we will appropriately kind our perform as:

def variable(*args: int, **kwargs: int) -> None:
    ...

This then passes kind checks:

$ mypy instance.py
Success: no points present in 1 supply file

Yay!

See Also

Fin

I’d wish to have an argument, please.

—Adam


Learn to make your exams run shortly in my ebook Speed Up Your Django Tests.


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Tags: mypy, python

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