Don't fail the entire right_hand_split if an optional split failed

Fixes splitting long import lines with only a single name.

.flake8

@@ -4,5 +4,5 @@
 [flake8]
 ignore = E203, E266, E501, W503
 max-line-length = 80
-max-complexity = 15
+max-complexity = 18
 select = B,C,E,F,W,T4,B9

README.md

@@ -547,6 +547,8 @@ More details can be found in [CONTRIBUTING](CONTRIBUTING.md).
 * fixed non-deterministic formatting when multiple pairs of removable parentheses
   were used (#183)
 
+* fixed not splitting long from-imports with only a single name
+
 
 ### 18.4a4
 

black.py

@@ -41,7 +41,7 @@
 from blib2to3.pgen2 import driver, token
 from blib2to3.pgen2.parse import ParseError
 
-__version__ = "18.4a5"
+__version__ = "18.4a6"
 DEFAULT_LINE_LENGTH = 88
 
 # types
@@ -1830,7 +1830,8 @@ def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
     """Split line into many lines, starting with the first matching bracket pair.
 
     Note: this usually looks weird, only use this for function definitions.
-    Prefer RHS otherwise.
+    Prefer RHS otherwise.  This is why this function is not symmetrical with
+    :func:`right_hand_split` which also handles optional parentheses.
     """
     head = Line(depth=line.depth)
     body = Line(depth=line.depth + 1, inside_brackets=True)
@@ -1870,7 +1871,10 @@ def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
 def right_hand_split(
     line: Line, py36: bool = False, omit: Collection[LeafID] = ()
 ) -> Iterator[Line]:
-    """Split line into many lines, starting with the last matching bracket pair."""
+    """Split line into many lines, starting with the last matching bracket pair.
+
+    If the split was by optional parentheses, attempt splitting without them, too.
+    """
     head = Line(depth=line.depth)
     body = Line(depth=line.depth + 1, inside_brackets=True)
     tail = Line(depth=line.depth)
@@ -1909,20 +1913,25 @@ def right_hand_split(
     bracket_split_succeeded_or_raise(head, body, tail)
     assert opening_bracket and closing_bracket
     if (
+        # the opening bracket is an optional paren
         opening_bracket.type == token.LPAR
         and not opening_bracket.value
+        # the closing bracket is an optional paren
         and closing_bracket.type == token.RPAR
         and not closing_bracket.value
+        # there are no delimiters or standalone comments in the body
+        and not body.bracket_tracker.delimiters
+        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)
+        and not line.is_import
     ):
-        # These parens were optional. If there aren't any delimiters or standalone
-        # comments in the body, they were unnecessary and another split without
-        # them should be attempted.
-        if not (
-            body.bracket_tracker.delimiters or line.contains_standalone_comments(0)
-        ):
-            omit = {id(closing_bracket), *omit}
+        omit = {id(closing_bracket), *omit}
+        try:
             yield from right_hand_split(line, py36=py36, omit=omit)
             return
+        except CannotSplit:
+            pass
 
     ensure_visible(opening_bracket)
     ensure_visible(closing_bracket)

tests/import_spacing.py

@@ -23,6 +23,7 @@
 from some_library import (
     Just, Enough, Libraries, To, Fit, In, This, Nice, Split, Which, We, No, Longer, Use
 )
+from name_of_a_company.extremely_long_project_name.component.ttypes import CuteLittleServiceHandlerFactoryyy
 
 from .a.b.c.subprocess import *
 from . import (tasks)
@@ -83,6 +84,9 @@
     Longer,
     Use,
 )
+from name_of_a_company.extremely_long_project_name.component.ttypes import (
+    CuteLittleServiceHandlerFactoryyy
+)
 
 from .a.b.c.subprocess import *
 from . import tasks