mirror of
https://github.com/jbranchaud/til
synced 2026-08-31 16:51:46 +00:00
62 lines
2.1 KiB
Markdown
62 lines
2.1 KiB
Markdown
# Force Remaining Arguments To Be Named
|
|
|
|
When the _positional argument collector_ (`*`) is used in a function definition,
|
|
all the arguments that follow it must now be explicitly named when invoking that
|
|
function. This is regardless of whether you are collecting positional arguments
|
|
into a named tuple (e.g. `*rest`) or are using the empty/dummy version (`*`).
|
|
|
|
A friend recently recommended I use this syntax to force some configuration
|
|
values (often booleans) to be explicitly named at call sites.
|
|
|
|
Here is an example of this in my [`py-vmt` project](https://github.com/jbranchaud/py-vmt):
|
|
|
|
```python
|
|
class CliContext:
|
|
def __init__(self, *, verbose: bool, repo: SessionRepository | None = None) -> None:
|
|
self.verbose: bool = verbose
|
|
self.active_session: Session | None = None
|
|
self.repo: SessionRepository = repo or JsonRepository()
|
|
self.active_session = self.repo.active_session()
|
|
```
|
|
|
|
Notice that right after `self` is the `*` which means that both `verbose` and
|
|
`repo` must be named arguments.
|
|
|
|
Here is a corresponding call site:
|
|
|
|
```python
|
|
ctx.obj = CliContext(verbose=True)
|
|
```
|
|
|
|
Now when I initialize `CliContext`, there is no choice for me to pass in an
|
|
unnamed argument of `True`. It has to be paired with `verbose`. Above I've
|
|
excluded `repo` because that falls back to a default value. If I were to include
|
|
it, it would have to be named.
|
|
|
|
The same applies to this scenario where I'm collecting the positional arguments.
|
|
|
|
```python
|
|
def build_identifier(first, *rest, delimiter="/"):
|
|
if rest is None:
|
|
return first
|
|
|
|
return delimiter.join([first, *rest])
|
|
```
|
|
|
|
```python
|
|
>>> print(build_identifier("taco"))
|
|
taco
|
|
>>> print(build_identifier("taco", "bell", "mas", delimiter="-"))
|
|
taco-bell-mas
|
|
>>> print(build_identifier("taco", "bell", "mas", "-"))
|
|
taco/bell/mas/-
|
|
```
|
|
|
|
Notice in the third example where I don't use a named argument for the
|
|
delimiter. It instead gets swept up in the collection of positional arguments.
|
|
|
|
This language of _positional argument collector_ I got from [Python in a
|
|
Nutshell, 4th
|
|
Edition](https://www.oreilly.com/library/view/python-in-a/9781098113544/) (see
|
|
pg. 96).
|