Split imports like isort

Fixes #127

Partially addresses #152

README.md

@@ -176,6 +176,13 @@ between two distinct sections of the code that otherwise share the same
 indentation level (like the arguments list and the docstring in the
 example above).
 
+If a line of "from" imports cannot fit in the allotted length, it's always split
+into one per line.  Imports tend to change often and this minimizes diffs, as well
+as enables readers of code to easily find which commit introduced a particular
+import.  This exception also makes *Black* compatible with
+[isort](https://pypi.org/p/isort/).  Use `multi_line_output=3` and
+`include_trailing_comma=True` in your isort config.
+
 
 ### Line length
 
@@ -528,6 +535,8 @@ More details can be found in [CONTRIBUTING](CONTRIBUTING.md).
 * Black no longer enforces putting empty lines behind control flow statements
   (#90)
 
+* Black now splits imports like "Mode 3 + trailing comma" of isort (#127)
+
 * fixed comment indentation when a standalone comment closes a block (#16, #32)
 
 * fixed standalone comments receiving extra empty lines if immediately preceding

black.py

@@ -1712,6 +1712,8 @@ def split_line(
     split_funcs: List[SplitFunc]
     if line.is_def:
         split_funcs = [left_hand_split]
+    elif line.is_import:
+        split_funcs = [explode_split]
     elif line.inside_brackets:
         split_funcs = [delimiter_split, standalone_comment_split, right_hand_split]
     else:
@@ -1978,6 +1980,24 @@ def append_to_line(leaf: Leaf) -> Iterator[Line]:
         yield current_line
 
 
+def explode_split(
+    line: Line, py36: bool = False, omit: Collection[LeafID] = ()
+) -> Iterator[Line]:
+    """Split by RHS and immediately split contents by a delimiter."""
+    new_lines = list(right_hand_split(line, py36, omit))
+    if len(new_lines) != 3:
+        yield from new_lines
+        return
+
+    yield new_lines[0]
+    try:
+        yield from delimiter_split(new_lines[1], py36)
+    except CannotSplit:
+        yield new_lines[1]
+
+    yield new_lines[2]
+
+
 def is_import(leaf: Leaf) -> bool:
     """Return True if the given leaf starts an import statement."""
     p = leaf.parent

tests/import_spacing.py

@@ -17,6 +17,10 @@
 from ..queues import *
 from ..streams import *
 
+from some_library import (
+    Just, Enough, Libraries, To, Fit, In, This, Nice, Split, Which, We, No, Longer, Use
+)
+
 from .a.b.c.subprocess import *
 from . import (tasks)
 from . import (A, B, C)
@@ -59,6 +63,23 @@
 from ..queues import *
 from ..streams import *
 
+from some_library import (
+    Just,
+    Enough,
+    Libraries,
+    To,
+    Fit,
+    In,
+    This,
+    Nice,
+    Split,
+    Which,
+    We,
+    No,
+    Longer,
+    Use,
+)
+
 from .a.b.c.subprocess import *
 from . import tasks
 from . import A, B, C