In python, how to tweak Black formatter, if possible?

This is now possible by adding a trailing comma to your last argument.

In your example you would write instead:

def example_function(
    arg_1: str, 
    arg_2: bool, 
    arg_3: int = 0, 
    arg_4: int = 1, 
    arg_5: float = 0.0, # <-- Notice the trailing comma
):

When using the Black playground at https://black.now.sh, I found that your function was reformatted exactly like that when the line length was short enough (in particular, at 78 characters).

It might be nice if there were a special configuration option that controlled the line length specifically for function parameter lines. BUT to seems to me that Black's configuration-free approach means that there is no way to control this with any more tailored option.

This is due to the default line length for black being longer than you'd like – 88 characters per line.

To decrease the line length, you can use the --line-length flag as documented here:

https://github.com/psf/black/blob/master/docs/installation_and_usage.md#command-line-options

For example:

$ black --line-length 80 example.py

Black explains the --line-length setting in more detail here:

https://github.com/psf/black/blob/master/docs/the_black_code_style.md#line-length

Line length

You probably noticed the peculiar default line length. Black defaults to 88 characters per line, which happens to be 10% over 80. This number was found to produce significantly shorter files than sticking with 80 (the most popular), or even 79 (used by the standard library). In general, 90-ish seems like the wise choice.

If you're paid by the line of code you write, you can pass --line-length with a lower number. Black will try to respect that. However, sometimes it won't be able to without breaking other rules. In those rare cases, auto-formatted code will exceed your allotted limit.

You can also increase it, but remember that people with sight disabilities find it harder to work with line lengths exceeding 100 characters. It also adversely affects side-by-side diff review on typical screen resolutions. Long lines also make it harder to present code neatly in documentation or talk slides.

Emphasis on the final paragraph.

I'd recommend just keeping the default settings. The beauty of Black is that it chooses for you, and therefor preempts any arguments about which way is "best".