Single-line match-cases with Ellipsis (...) as body

[pfaion]

Thanks for the great work on ruff!

Not sure if this a bug, probably rather a feature request. Curious to hear what you think about this:

I work with complexly nested data and sometimes I want to extract a value from potentially different parts of the data, depending on the structure of the data. I realized that you can quite nicely use "structural pattern matching" for that, e.g. here's a simple example:

class User(BaseModel):
    global_name: str
    # ...

class Member(BaseModel):
    nickname: Optional[str]
    user: User
    # ...

name: str
match member:
    case Member(nickname=str(name)): ...
    case Member(user=User(global_name=str(name))): ...
    
# Now `name` is bound, depending on which structure first "resolved". If the member had
# a nickname defined, we prefer using that. Otherwise we use the global_name.

I wrote the single-line case statements similar to how you can write single-line function signatures for e.g. Protocols (e.g. refer to #10026). However, ruff will always reformat them to:

name: str
match member:
    case Member(nickname=str(name)):
        ...
    case Member(user=User(global_name=str(name))):
        ...

I was wondering if case-statements should also allowed to be single-line if the body is an Ellipsis ... for consistency?

[MichaReiser]

I'm open to this but I don't think it should be specific to match cases. Instead, it is a more general question if ... bodies should be collapsed in compound statements.

[pfaion]

What other cases would there be? Any statement ending with a :?

First brainstorm:

if expr: ...
elif expr: ...
else: ...

while expr: ...

for val in expr: ...

class Foo: ...

def foo(): ...
async def foo(): ...

match expr: ...
    case pattern: ...

try: ...
except: ...
finally: ...

with expr as val: ...
async with expr as val: ...