Flags with `multiple=True` trigger `TypeError`

[amyreese]

Adding an option with is_flag=True and multiple=True will trigger a TypeError in Parameter.consume_value() due to bad default value.

import click

@click.command()
@click.option("-v", is_flag=True, multiple=True)
def main(v):
    click.echo(v)

if __name__ == "__main__":
    main()

When run without the flag, a TypeError is raised:

(venv39) jreese@butterfree ~/scratch » python foo.py
Traceback (most recent call last):
  File "/Users/jreese/scratch/foo.py", line 25, in <module>
    main()
  File "/Users/jreese/scratch/venv39/lib/python3.9/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/Users/jreese/scratch/venv39/lib/python3.9/site-packages/click/core.py", line 1054, in main
    with self.make_context(prog_name, args, **extra) as ctx:
  File "/Users/jreese/scratch/venv39/lib/python3.9/site-packages/click/core.py", line 920, in make_context
    self.parse_args(ctx, args)
  File "/Users/jreese/scratch/venv39/lib/python3.9/site-packages/click/core.py", line 1378, in parse_args
    value, args = param.handle_parse_result(ctx, opts, args)
  File "/Users/jreese/scratch/venv39/lib/python3.9/site-packages/click/core.py", line 2356, in handle_parse_result
    value, source = self.consume_value(ctx, opts)
  File "/Users/jreese/scratch/venv39/lib/python3.9/site-packages/click/core.py", line 2903, in consume_value
    and any(v is _flag_needs_value for v in value)
TypeError: 'bool' object is not iterable

Manually setting a better default value, like default=(), avoids the type error, but this used to work on 7.x without setting the default value.

Environment:

• Python version: tested on 3.8.6 and 3.9.3
• Click version: worked on 7.1.2; errors on 8.0.4 and 8.1.2

[davidism]

I can't remember what the original discussion about this was, but it seems like this should have been addressed by modifying the default default for multiple=True, flag=True rather than raising an error. The following works fine, the default=() should be implicit.

import click

@click.command
@click.option(
    "-y/-n",
    is_flag=True,
    multiple=True,
    default=()
)
def main(y):
    click.echo(y)


if __name__ == "__main__":
    main([])

[davidism]

Issues #2292 and #2295 are due to the error added by the PR for this issue.