Make sure async for is not broken up to separate lines (#503)

Fixes #372.

README.md

@@ -872,6 +872,8 @@ More details can be found in [CONTRIBUTING](CONTRIBUTING.md).
 
 * fixed formatting of lambda expressions with default arguments (#468)
 
+* *Black* no longer breaks ``async for`` statements up to separate lines (#372)
+
 
 ### 18.6b4
 

black.py

@@ -1889,7 +1889,7 @@ def is_split_after_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
 
 
 def is_split_before_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
-    """Return the priority of the `leaf` delimiter, given a line before after it.
+    """Return the priority of the `leaf` delimiter, given a line break before it.
 
     The delimiter priorities returned here are from those delimiters that would
     cause a line break before themselves.
@@ -1926,15 +1926,20 @@ def is_split_before_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
     ):
         return STRING_PRIORITY
 
-    if leaf.type != token.NAME:
+    if leaf.type not in {token.NAME, token.ASYNC}:
         return 0
 
     if (
         leaf.value == "for"
         and leaf.parent
         and leaf.parent.type in {syms.comp_for, syms.old_comp_for}
+        or leaf.type == token.ASYNC
     ):
-        return COMPREHENSION_PRIORITY
+        if (
+            not isinstance(leaf.prev_sibling, Leaf)
+            or leaf.prev_sibling.value != "async"
+        ):
+            return COMPREHENSION_PRIORITY
 
     if (
         leaf.value == "if"

tests/data/python37.py

@@ -3,6 +3,17 @@
 def f():
     return (i*2 async for i in arange(42))
 
+def g():
+    return (something_long * something_long async for something_long in async_generator(with_an_argument))
+
+async def func():
+    if test:
+        out_batched = [
+            i
+            async for i in aitertools._async_map(
+                self.async_inc, arange(8), batch_size=3
+            )
+        ]
 # output
 
 
@@ -11,3 +22,20 @@ def f():
 
 def f():
     return (i * 2 async for i in arange(42))
+
+
+def g():
+    return (
+        something_long * something_long
+        async for something_long in async_generator(with_an_argument)
+    )
+
+
+async def func():
+    if test:
+        out_batched = [
+            i
+            async for i in aitertools._async_map(
+                self.async_inc, arange(8), batch_size=3
+            )
+        ]