Variable precedence

[rgreinho]

Playing with click's ability to read environment variables and provide default values, I noticed that the order in which the variables are looked up seems wrong.

Here is what I expected (first variable found wins):

1. Retrieve the value from the CLI
2. Retrieve the value from the environment
3. Retrieve the default value

But here is what happened:

1. Retrieve the value from the CLI
2. Retrieve the default value
3. Retrieve the value from the environment

This seems wrong because the fact that I set default values should not prevent the environment lookup to happen.

How to reproduce

Create the precedence.py file:

import click

@click.command()
@click.option('--name', help="Specify a name")
def prec(name):
    """Greet the world."""
    click.echo(f'Hey {name}!')


if __name__ == '__main__':
    prec(auto_envvar_prefix='CLICK', default_map={'name': 'Stranger'})

Then run:

$ export CLICK_NAME=Tiger
$ python precedence.py --name Joe
Hey Joe!
$ python precedence.py
Hey Stranger!

Then remove the default_map argument from the command call and you get:

$ export CLICK_NAME=Tiger
$ python precedence.py --name Joe
Hey Joe!
$ python precedence.py
Hey Tiger!

Versions

Python: 3.6.3
Click: 6.7

[chaoflow]

I also see this as an issue. Quoting the documentation: "By default, the default value for a parameter is pulled from the default flag that is provided when it’s defined, but that’s not the only place defaults can be loaded from."

Currently, a default value provided via default_map takes precedence over environment variables, whereas environment variables take precedence over default values provided to click.option().