A comma should only be considered magical if followed by a newline

[graingert]

I think there's something here. A comma should only be considered magical if followed by a newline

Originally posted by @graingert in #2353 (comment)

[graingert]

Eg I think:
Before

x = {'a':42}
y = {'a':42,}
z = {'a':42,
}

After

x = {'a':42}
y = {'a':42}
z = {
    'a':42,
}

Is more understandable behaviour for black

[graingert]

Or more specifically any whitespace including newlines before any other character

[felix-hilden]

This actually seems pretty reasonable, although I haven't really thought it through carefully.

[graingert]

I just found a counter-example:

            self.lifespan_context = _wrap_gen_lifespan_context(
                lifespan,  # type: ignore[arg-type]
            )

[graingert]

so I think a narrower definition is to define a comma followed by a paren, bracket or brace as non-magical

[graingert]

eg:

# magic:
            self.lifespan_context = _wrap_gen_lifespan_context(
                lifespan,  # type: ignore[arg-type]
            )

# magic:
            self.lifespan_context = _wrap_gen_lifespan_context(
                lifespan,  # type: ignore[arg-type])

# maybe not magic:
            self.lifespan_context = _wrap_gen_lifespan_context(
                lifespan,  )

# probably not magic:
            self.lifespan_context = _wrap_gen_lifespan_context(
                lifespan,)

# not magic:
            self.lifespan_context = _wrap_gen_lifespan_context(lifespan,     )

# not magic:
            self.lifespan_context = _wrap_gen_lifespan_context(lifespan,)

[hauntsaninja]

It should be easy for humans to simulate Black's formatting, and I think this proposal crosses a complexity boundary. The magic trailing comma is already pretty magical and paying even more attention to existing formatting introduces more implementation complexity.