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Bug: Optional value not optional anymore #3084
Description
Outline
As stated in official documentation:
Setting
is_flag=False, flag_value=valuetells Click that the option can still be passed a value, but if only the flag is given, the value will beflag_value.
Since 8.3.0 this doesn't work anymore, instead SystemExit with "Error: Option '--xyz' requires an argument." is thrown.
Replication of bug
Example taken from documentation:
@click.command()
@click.option("--name", is_flag=False, flag_value="Flag", default="Default")
def hello(name):
click.echo(f"Hello, {name}!")$ hello --name
Error: Option '--name' requires an argument.Expected behavior
The code example above should work as
$ hello --name
Hello, Flag!It's not even working in the generated documentation anymore: while documentation up to Sep, 10th shows the expected and correct behavior, current documentation demonstrates the bug.
- Python version: 3.11
- Click version: 8.3.0
Reacted by djkawa, Chris Wilson, Helge, Doug Richardson, Sewer., Dave Hirschfeld, Akshath Kothari, Y.D.X., John Hagen, Jort Koopmans and 4 more
Activity
Rowlando13 commentedon Sep 24, 2025
Thanks for the clear report! We will work on this
pawamoy commentedon Oct 2, 2025
MkDocs seems to be affected by this again, like with version 8.2.2. Here's how it declares its boolean flags: https://github.com/mkdocs/mkdocs/blob/f68e5fc18d3cd3811dbddfdeda1a57721f9e2e4e/mkdocs/__main__.py#L253-L264. Should MkDocs change something or is Click 8.3.0 suffering from the same kind of issue as 8.2.2?
Rowlando13 commentedon Oct 2, 2025
This is still broken in 8.3.0.
Fix pallets#3084: Optional value not working with flag_value
Fix pallets#3084: Correct flag optional value behavior and add compre…
Fix pallets#3084: Restore documented flag_value behavior, update docs…
Fix pallets#3084: Correct flag optional value behavior and add compre…
chore: Downgrade click to mitigate a temporary issue for mkdocs serve…
docs: workaround upstream mkdocs live reloading bug
Upgrade Inspect AI to 0.3.146.dev9+gdeec1b7a (#568)
uriii3 commentedon Nov 19, 2025
Fix pallets#3084: Correct flag optional value behavior and add compre…
is_flag=Falseis broken #3336 as a duplicate of this issue