Why can't I find a single example of validating string input?

[TheKayThatWasOrange]

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 tutorials 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

def main(preferred_servers: list[str]):

    print(preferred_servers)
    return

Description

I feel like I've been searching for this for way too long and should just go back to argparse if it's really not a practical use case for typer.

I want to pass a list of strings to my tool.

I want to verify that those strings match against some arbitrary pattern--say IP addresses or email addresses or something else that absolutely cannot be represented by an enum.

If those strings do not match against my arbitrary pattern, I want to print some useful error text.

I see that I can define formatters for datetimes. I don't see an obvious means of applying that option to a list of strings or supplying it with a regex so it seems like a non-starter.

I see that I can define custom types and it's conceivable that I could design a class which takes a string in its construction and raises an exception if that string doesn't fit its expectations, but what exception would be appropriate? What should that look like for a list of those custom objects? Should my custom object actually be the list itself?

If I'm being stupid and missing something obvious please let me know as I feel like I've read the phrase "you'll learn more about validations later" over and over but never actually learned anything whatsoever.

Operating System

macOS

Operating System Details

No response

Typer Version

0.25.1

Python Version

3.13.2

Additional Context

No response

[svlandeg]

Hi! You should be able to accomplish what you want quite easily with a parameter callback, see https://typer.tiangolo.com/tutorial/options/callback-and-context/#validate-cli-parameters

(the example in the docs there is literally one around validating string input)

[TheKayThatWasOrange]

Pretty close, but I'm not seeing how that would help with a list of strings. If I can only attach one callback to the list then I'm ultimately still writing some enumeration out longhand--rejecting the entire list as invalid when in reality only a subset of elements in it might be bad--and I'd just assume have the brute force approach be blatantly obvious than hidden under some metaprogramming magic rocks.

Disappointing, but it is what it is.

[MaxFreedomPollard]

A parameter callback does give you per-element validation. The trick is that the callback receives the whole list, so you loop over it, collect the elements that fail, and raise typer.BadParameter naming exactly which ones were bad. You're not rejecting the list as an opaque blob, and you're not enumerating anything longhand.

import re
from typing import List
import typer

IPV4 = re.compile(r"^(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})$")

def valid_ip(s: str) -> bool:
    m = IPV4.match(s)
    return bool(m) and all(0 <= int(g) <= 255 for g in m.groups())

def validate_servers(value: List[str]) -> List[str]:
    bad = [v for v in value if not valid_ip(v)]
    if bad:
        raise typer.BadParameter(f"not valid IPv4: {', '.join(bad)}")
    return value

app = typer.Typer()

@app.command()
def main(server: List[str] = typer.Option(None, callback=validate_servers)):
    typer.echo(f"ok: {server}")

Run it with a couple of bad entries and you get a precise message, not a blanket rejection:

$ app --server 10.0.0.1 --server 999.1.2.3 --server nope
Error
Invalid value for '--server': not valid IPv4: 999.1.2.3, nope

The callback returns the validated list, so server inside your command is exactly what you'd expect. Swap the regex for an email pattern or whatever else you need.

Two variations depending on what you actually want:

• Reject the whole run but tell the user which entries were wrong: raise typer.BadParameter as above. This is the usual CLI behavior.
• Keep the good ones and drop the bad: don't raise, just return [v for v in value if valid_ip(v)] (and print a warning for the dropped ones if you like).

On the custom-type idea: you can write a click.ParamType whose convert() validates one value and calls self.fail(...), and Typer applies it to each element of the list, which gives you one error per bad element. One caveat, recent Typer bundles its own Click and no longer depends on the standalone click package, so import click may not resolve unless you install it explicitly. The callback above needs no extra dependency, which is why I'd reach for it first.

Tested on Typer 0.26.8, Python 3.11.