Fix unnecessary parentheses when a line contains multiline strings

Fixes psf#232

README.md

@@ -721,6 +721,8 @@ More details can be found in [CONTRIBUTING](CONTRIBUTING.md).
 
 * fixed long trivial assignments being wrapped in unnecessary parentheses (#273)
 
+* fixed unnecessary parentheses when a line contained multiline strings (#232)
+
 * fixed stdin handling not working correctly if an old version of Click was
  used (#276)
 

black.py

@@ -1094,6 +1094,13 @@ def contains_standalone_comments(self, depth_limit: int = sys.maxsize) -> bool:
 
  return False
 
+ def contains_multiline_strings(self) -> bool:
+ for leaf in self.leaves:
+ if is_multiline_string(leaf):
+ return True
+
+ return False
+
  def maybe_remove_trailing_comma(self, closing: Leaf) -> bool:
  """Remove trailing comma if there is one and it's safe."""
  if not (
@@ -2225,24 +2232,35 @@ def right_hand_split(
  # the closing bracket is an optional paren
  and closing_bracket.type == token.RPAR
  and not closing_bracket.value
- # there are no standalone comments in the body
- and not line.contains_standalone_comments(0)
- # and it's not an import (optional parens are the only thing we can split
- # on in this case; attempting a split without them is a waste of time)
+ # it's not an import (optional parens are the only thing we can split on
+ # in this case; attempting a split without them is a waste of time)
  and not line.is_import
+ # there are no standalone comments in the body
+ and not body.contains_standalone_comments(0)
+ # and we can actually remove the parens
+ and can_omit_invisible_parens(body, line_length)
  ):
  omit = {id(closing_bracket), *omit}
- if can_omit_invisible_parens(body, line_length):
- try:
- yield from right_hand_split(line, line_length, py36=py36, omit=omit)
- return
- except CannotSplit:
- if len(body.leaves) == 1 and not is_line_short_enough(
- body, line_length=line_length
- ):
- raise CannotSplit(
- "Splitting failed, body is still too long and can't be split."
- )
+ try:
+ yield from right_hand_split(line, line_length, py36=py36, omit=omit)
+ return
+
+ except CannotSplit:
+ if not (
+ can_be_split(body)
+ or is_line_short_enough(body, line_length=line_length)
+ ):
+ raise CannotSplit(
+ "Splitting failed, body is still too long and can't be split."
+ )
+
+ elif head.contains_multiline_strings() or tail.contains_multiline_strings():
+ raise CannotSplit(
+ "The current optional pair of parentheses is bound to fail to "
+ "satisfy the splitting algorithm becase the head or the tail "
+ "contains multiline strings which by definition never fit one "
+ "line."
+ )
 
  ensure_visible(opening_bracket)
  ensure_visible(closing_bracket)
@@ -3190,6 +3208,42 @@ def is_line_short_enough(line: Line, *, line_length: int, line_str: str = "") ->
  )
 
 
+def can_be_split(line: Line) -> bool:
+ """Return False if the line cannot be split *for sure*.
+
+ This is not an exhaustive search but a cheap heuristic that we can use to
+ avoid some unfortunate formattings (mostly around wrapping unsplittable code
+ in unnecessary parentheses).
+ """
+ leaves = line.leaves
+ if len(leaves) < 2:
+ return False
+
+ if leaves[0].type == token.STRING and leaves[1].type == token.DOT:
+ call_count = 0
+ dot_count = 0
+ next = leaves[-1]
+ for leaf in leaves[-2::-1]:
+ if leaf.type in OPENING_BRACKETS:
+ if next.type not in CLOSING_BRACKETS:
+ return False
+
+ call_count += 1
+ elif leaf.type == token.DOT:
+ dot_count += 1
+ elif leaf.type == token.NAME:
+ if not (next.type == token.DOT or next.type in OPENING_BRACKETS):
+ return False
+
+ elif leaf.type not in CLOSING_BRACKETS:
+ return False
+
+ if dot_count > 1 and call_count > 1:
+ return False
+
+ return True
+
+
 def can_omit_invisible_parens(line: Line, line_length: int) -> bool:
  """Does `line` have a shape safe to reformat without optional parens around it?
 

tests/cantfit.py

@@ -28,6 +28,13 @@
 string_variable_name = (
  "a string that is waaaaaaaayyyyyyyy too long, even in parens, there's nothing you can do" # noqa
 )
+for key in """
+ hostname
+ port
+ username
+""".split():
+ if key in self.connect_kwargs:
+ raise ValueError(err.format(key))
 
 
 # output
@@ -71,3 +78,10 @@
  this_is_a_ridiculously_long_name_and_nobody_in_their_right_mind_would_use_one_like_it=0,
 )
 string_variable_name = "a string that is waaaaaaaayyyyyyyy too long, even in parens, there's nothing you can do" # noqa
+for key in """
+ hostname
+ port
+ username
+""".split():
+ if key in self.connect_kwargs:
+ raise ValueError(err.format(key))