Detect unintended comma at the end of expression

[xuhcc]

I would be great to have a rule that will detect cases such this:

instance = MyClass(arg1, arg2),

As it is most likely a typo.
If someone wants a tuple, the preferred form would be (item,).

[ambv]

That would be neat, indeed. Sadly, it's not trivial since our AST doesn't include this information.

[carljm]

It is possible, though, since flake8 can give access to the raw source, and then can use Python's parser module to get a parse tree and work on that.

[ambv]

I'm only worried about the additional overhead for just this one check. lib2to3 is in Python so it's going to slow down Bugbear a bit. We can do it and measure how bad that slowdown is but I have a feeling this is a better job for Black to always insert parentheses around single-element tuples to make this obvious.

[gaul]

Not to deter adding this to bugbear, but flake8_tuple finds unintended tuples.

[ambv]

If you use http://github.com/ambv/black/, it will helpfully wrap one-tuples in parentheses to make this more obvious. So if this wasn't intentional, it will be easier to spot.

[DylanYoung]

https://pypi.org/project/flake8-commas/ is another option.