default=lambda and show_default behavior

[jcejohnson]

This:
@click.option('--username', callback=set_option_value, default=lambda: get_option_default('username'), show_default=True)

gives this --help output:
--username TEXT [default: <function <lambda> at 0x104858a28>]

If that is the intended behavior, is there an option to have it invoke the function during --help?

[untitaker]

The current behavior is a bug, but I'm unsure whether it's a good idea to just inline the return value of the lambda? IMO it's better to describe the default callback's logic in the option helptext.

[jcejohnson]

I think I agree but it would also be nice to have the option for --help to invoke the function.

[blueyed]

JFI: the code in question:

click/click/core.py

Lines 1635 to 1639 in defe6ae

	if self.default is not None and self.show_default:
	extra.append('default: %s' % (
	', '.join('%s' % d for d in self.default)
	if isinstance(self.default, (list, tuple))
	else self.default, ))

[dougharris]

In #844, the suggestion is made to evaluate the default value when running --help. This seems possibly dangerous or ripe for exploitation if the lambda on the option ran something which updates a database, writes to a file, etc.

Instead, I'm working on a fix for this that adds a new optional parameter to @click.option to be used to give a description of what the dynamic value would be.