Typer expects default value for CustomClass as string instead of class itself, which breaks typing and breaks using this function without typer

[AlexeyDmitriev]

First Check

• I added a very descriptive title here.
• I used the GitHub search to find a similar question and didn't find it.
• I searched the Typer documentation, with the integrated search.
• I already searched in Google "How to X in Typer" and didn't find any information.
• I already read and followed all the tutorial in the docs and didn't find an answer.
• I already checked if it is not related to Typer but to Click.

Commit to Help

• I commit to help with one of those options 👆

Example Code

import typer
import datetime
from typing_extensions import Annotated


def parse(s: str):
    return datetime.datetime.strptime(s, "%Y-%m-%d").date()

def main(date: Annotated[datetime.date, typer.Option(parser=parse)] = "2023-12-18"):
    print(date)

if __name__ == '__main__':
    typer.run(main)

Description

So example code above works fine, but, it doesn't type check properly because main supposed to accept datetime.date, and it's default is string.
Also, if I try to run function main without typer(a big feature for typer that it's usually possible), this argument will get str

If I change it to

import typer
import datetime
from typing_extensions import Annotated


def parse(s: str):
    return datetime.datetime.strptime(s, "%Y-%m-%d").date()

def main(date: Annotated[datetime.date, typer.Option(parser=parse)] = datetime.date(2023, 12, 18)):
    print(date)

if __name__ == '__main__':
    typer.run(main)

which type-checks fine, I'll get TypeError: strptime() argument 1 must be str, not datetime.date because typer expects that I give it a string, not datetime and calls parser on it

Operating System

Linux, Windows, macOS, Other

Operating System Details

any

Typer Version

0.9.0

Python Version

Python 3.11.4

Additional Context

I think I'm ready to (try to) contribute this as PR, if this is considered worth fixing

[ambroisie]

I was recently bitten by a similar issue, providing default_factory=MyClass and parser=MyClass (where MyClass can be built without arguments, or with one string positional argument) leads to typer trying to call MyClass(MyClass()) (the outer one is the parser, the inner one the default value).

[svlandeg]

I can see how this construction is preventing a proper type check, as the parser is being applied to the parameter value, whether it is user-defined or default, which means the default needs to be a str and not a CustomClass 🤔

I think I'm ready to (try to) contribute this as PR, if this is considered worth fixing

How would you attempt to fix this?

[AlexeyDmitriev]

How would you attempt to fix this?

So I see 3 main ways to fix it

1. Instead of FuncParamType https://github.com/pallets/click/blob/8.1.x/src/click/types.py implement another ParamType that
   will first check if it's given str or the type and if it's a type then return the same value, without trying to parse it.

I'm pretty sure this will work, even if it feels a bit dirty.

2. To look how exactly typer and click interact here and ensure that in case of default, click doesn't interact here
   For example, one possible way is to have optional string click parameter and postprocess yourself only if needed (if nothing is passed, return default otherwise call parser(given_string_value)

I'm not sure it's doable without complete rewrite of how typer works (not deep enough in this code)

3. Rewrite how click uses FuncParamType so that this requirement is not needed for them
   https://github.com/pallets/click/blob/8.1.x/src/click/types.py#L34

Also not sure how feasible is that.

If I'd make a PR, I'd do the first way.