Odd Formatting Bug

[ghost]

To Reproduce Steps to reproduce the behavior:
Write this:

posts = [
    {
        "author": "CoreyMS",
        "title": "Blog Post 1",
        "content": "First post content",
        "date_posted": datetime.datetime.utcnow().strftime("%A, %x")
    },
    {
        "author": "CoreyMS",
        "title": "Blog Post 1",
        "content": "First post content",
        "date_posted": datetime.datetime.utcnow() + datetime.timedelta(3).strftime("%A, %x"),
    }
]

Press ctrl + s to save on vscode with black as the formatter (It does not matter if you do this or run black normally with black .
This happens:

posts = [
    {"author": "CoreyMS", "title": "Blog Post 1", "content": "First post content", "date_posted": datetime.datetime.utcnow().strftime("%A, %x")},
    {
        "author": "CoreyMS",
        "title": "Blog Post 1",
        "content": "First post content",
        "date_posted": datetime.datetime.utcnow() + datetime.timedelta(3).strftime("%A, %x"),
    },
]

Expected behavior:
It keeps that style instead of the (no-offence) ugly formatting

Environment:

• Version: Master
• OS and Python version: Kubuntu 20.04 LTS focal fossa (kde 5) with python 3.8.2

Does this bug also happen on master?
It does

Additional context:
Also with this, black is colliding with flake8. Black makes a code that looks like this:

example = {'one': 'first', 'two': 'second'}

To this:

example = {'one': 'first', 'two': 'second',}

(It adds the extra , at the end without a space)
This raises missing whitespace after ','flake8(E231)
Is there a configuration option to either add a space or just not have a trailing comma at the end? I tried to look up in the docs for such configuration option but failed to find it 😞
Additionally, I cannot see any configuration options in the docs that I can add in the configuration file

[mx-psi]

If you add a trailing comma at the end of the date_posted field it should keep it as it is. The behavior is being worked on (see #1288) but that should work for now.

[ghost]

I tried to play with the styling on this dictionary and this is what I wanted:

But when I format it I get this:

Adding a trailing comma also does not fix this 😞

[mx-psi]

Maybe I explained myself incorrectly. In your screenshots you are removing the trailing comma and adding one trailing comma at the end, while what I suggested was doing:

posts = [
    {
        "author": "CoreyMS",
        "title": "Blog Post 1",
        "content": "First post content",
        "date_posted": datetime.datetime.utcnow().strftime("%A, %x"),
    },
    {
        "author": "CoreyMS",
        "title": "Blog Post 1",
        "content": "First post content",
        "date_posted": datetime.datetime.utcnow() + datetime.timedelta(3).strftime("%A, %x"),
    }
]

which should keep the format.

[ghost]

Nothing changes, even if I copy the same exact code that you sent 😞

[ichard26]

For clarity, the "magic trailing comma" feature doesn't work in nested collection literals yet. This falls under the scope of #1288 and there is already an issue filed for this bug: #1377.