I can confirm Guido's words, now parentheses for continuation across lines are already supported.

Even without parentheses, multiline with items can be supported. I just implemented it here: https://github.com/thautwarm/cpython/blob/bpo-12782/Grammar/python.gram#L180-L187

  from contextlib import contextmanager

  @contextmanager
  def f(x):
    try:
        yield x
    finally:
        pass

# Ok
  with f('c') as a,
       f('a') as b:
       pass


# Ok
  with f('c') as a,
       f('a') as b,
       f('a') as c:
       pass


  # ERROR
  with f('c') as a,
       f('a') as b,
       f('a') as c:
     x = 1 + 1

  # message:
    File "/home/thaut/github/cpython/../a.py", line 49
      x = 1 + 1
             ^
    IndentationError: unindent does not match any outer indentation 
level

  # ERROR
  with f('c') as a,
       f('a') as b,
       f('a') as c:
          x = 1 + 1

  File "/home/thaut/github/cpython/../a.py", line 49
    x = 1 + 1
  
  IndentationError: unexpected indent



The grammar is:


with_stmt[stmt_ty]:
    | ...
    | 'with' a=(',' [NEWLINE ~ INDENT?]).with_item+ ':' tc=[TYPE_COMMENT] NEWLINE b=statements DEDENT {
        _Py_With(a, b, NEW_TYPE_COMMENT(p, tc), EXTRA) }
    | ...

The restriction here is, since the second 'with_item', until the end of 'statements', the expression and statements have to keep the same indentation.

    with item1,
       item2,
       ...:
       block
    
    The indentation of 'item2', ..., 'block' should be the same.

This implementation leverages the new PEG and how the lexer deals with indent/dedent.