Repository: psf/black (Python)

Issue:
Lines should break before all binary operators
Style of line breaks is not consistent.

```python
(
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa |
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
)

(
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa &
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
)

(
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa -
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
)

(
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    and aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
)

(
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    or aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
)
```

According to [PEP8](https://www.python.org/dev/peps/pep-0008/#should-a-line-break-before-or-after-a-binary-operator), lines should break before all binary operators.


Discussion:
Agreed.
@Autophagy, would you be willing to move this further so that all delimiters are split to be at the beginning of the line? You could refactor the code to make this the default and *only* do the opposite thing for commas.
@ambv Totally! That's a great idea :sparkles:
Alright, awaiting your pull request. Don't forget to add yourself to "Authors" in the README this time 😉 



Patch:
diff --git a/README.md b/README.md
index 4e3ad33daca..3ab68878c44 100644
--- a/README.md
+++ b/README.md
@@ -385,6 +385,9 @@ More details can be found in [CONTRIBUTING](CONTRIBUTING.md).
 
 ### 18.3a5 (unreleased)
 
+* add line breaks before all delimiters, except in cases like commas, to better
+  comply with PEP8 (#73)
+
 * fixed handling of standalone comments within nested bracketed
   expressions; Black will no longer produce super long lines or put all
   standalone comments at the end of the expression (#22)
@@ -504,5 +507,5 @@ Multiple contributions by:
 * [Daniel M. Capella](mailto:polycitizen@gmail.com)
 * [Eli Treuherz](mailto:eli.treuherz@cgi.com)
 * Hugo van Kemenade
-* [Mika⠙](mailto:mail@autophagy.io)
+* [Mika Naylor](mailto:mail@autophagy.io)
 * [Osaetin Daniel](mailto:osaetindaniel@gmail.com)
diff --git a/black.py b/black.py
index dc03e0a9b3a..3d5ea14736e 100644
--- a/black.py
+++ b/black.py
@@ -451,6 +451,7 @@ def show(cls, code: str) -> None:
     token.DOUBLESTAR,
     token.DOUBLESLASH,
 }
+VARARGS = {token.STAR, token.DOUBLESTAR}
 COMPREHENSION_PRIORITY = 20
 COMMA_PRIORITY = 10
 LOGIC_PRIORITY = 5
@@ -492,32 +493,12 @@ def mark(self, leaf: Leaf) -> None:
             leaf.opening_bracket = opening_bracket
         leaf.bracket_depth = self.depth
         if self.depth == 0:
-            delim = is_delimiter(leaf)
-            if delim:
-                self.delimiters[id(leaf)] = delim
-            elif self.previous is not None:
-                if leaf.type == token.STRING and self.previous.type == token.STRING:
-                    self.delimiters[id(self.previous)] = STRING_PRIORITY
-                elif (
-                    leaf.type == token.NAME
-                    and leaf.value == "for"
-                    and leaf.parent
-                    and leaf.parent.type in {syms.comp_for, syms.old_comp_for}
-                ):
-                    self.delimiters[id(self.previous)] = COMPREHENSION_PRIORITY
-                elif (
-                    leaf.type == token.NAME
-                    and leaf.value == "if"
-                    and leaf.parent
-                    and leaf.parent.type in {syms.comp_if, syms.old_comp_if}
-                ):
-                    self.delimiters[id(self.previous)] = COMPREHENSION_PRIORITY
-                elif (
-                    leaf.type == token.NAME
-                    and leaf.value in LOGIC_OPERATORS
-                    and leaf.parent
-                ):
-                    self.delimiters[id(self.previous)] = LOGIC_PRIORITY
+            after_delim = is_split_after_delimiter(leaf, self.previous)
+            before_delim = is_split_before_delimiter(leaf, self.previous)
+            if after_delim > before_delim:
+                self.delimiters[id(leaf)] = after_delim
+            elif before_delim > after_delim and self.previous is not None:
+                self.delimiters[id(self.previous)] = before_delim
         if leaf.type in OPENING_BRACKETS:
             self.bracket_match[self.depth, BRACKET[leaf.type]] = leaf
             self.depth += 1
@@ -1374,17 +1355,35 @@ def preceding_leaf(node: Optional[LN]) -> Optional[Leaf]:
     return None
 
 
-def is_delimiter(leaf: Leaf) -> int:
-    """Return the priority of the `leaf` delimiter. Return 0 if not delimiter.
+def is_split_after_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
+    """Return the priority of the `leaf` delimiter, given a line break after it.
+
+    The delimiter priorities returned here are from those delimiters that would
+    cause a line break after themselves.
 
     Higher numbers are higher priority.
     """
     if leaf.type == token.COMMA:
         return COMMA_PRIORITY
 
-    if leaf.type in COMPARATORS:
-        return COMPARATOR_PRIORITY
+    if (
+        leaf.type in VARARGS
+        and leaf.parent
+        and leaf.parent.type in {syms.argument, syms.typedargslist}
+    ):
+        return MATH_PRIORITY
+
+    return 0
 
+
+def is_split_before_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
+    """Return the priority of the `leaf` delimiter, given a line before after it.
+
+    The delimiter priorities returned here are from those delimiters that would
+    cause a line break before themselves.
+
+    Higher numbers are higher priority.
+    """
     if (
         leaf.type in MATH_OPERATORS
         and leaf.parent
@@ -1392,9 +1391,49 @@ def is_delimiter(leaf: Leaf) -> int:
     ):
         return MATH_PRIORITY
 
+    if leaf.type in COMPARATORS:
+        return COMPARATOR_PRIORITY
+
+    if (
+        leaf.type == token.STRING
+        and previous is not None
+        and previous.type == token.STRING
+    ):
+        return STRING_PRIORITY
+
+    if (
+        leaf.type == token.NAME
+        and leaf.value == "for"
+        and leaf.parent
+        and leaf.parent.type in {syms.comp_for, syms.old_comp_for}
+    ):
+        return COMPREHENSION_PRIORITY
+
+    if (
+        leaf.type == token.NAME
+        and leaf.value == "if"
+        and leaf.parent
+        and leaf.parent.type in {syms.comp_if, syms.old_comp_if}
+    ):
+        return COMPREHENSION_PRIORITY
+
+    if leaf.type == token.NAME and leaf.value in LOGIC_OPERATORS and leaf.parent:
+        return LOGIC_PRIORITY
+
     return 0
 
 
+def is_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
+    """Return the priority of the `leaf` delimiter. Return 0 if not delimiter.
+
+    Higher numbers are higher priority.
+    """
+    return max(
+        is_split_before_delimiter(leaf, previous),
+        is_split_after_delimiter(leaf, previous),
+    )
+
+
 def generate_comments(leaf: Leaf) -> Iterator[Leaf]:
     """Clean the prefix of the `leaf` and generate comments from it, if any.
 
diff --git a/docs/reference/reference_functions.rst b/docs/reference/reference_functions.rst
index c167c4b9a00..b521f789e4a 100644
--- a/docs/reference/reference_functions.rst
+++ b/docs/reference/reference_functions.rst
@@ -12,6 +12,10 @@ Assertions and checks
 
 .. autofunction:: black.assert_stable
 
+.. autofunction:: black.is_split_after_delimiter
+
+.. autofunction:: black.is_split_before_delimiter
+
 .. autofunction:: black.is_delimiter
 
 .. autofunction:: black.is_import


Test patch:
diff --git a/tests/expression.py b/tests/expression.py
index 1e8fa5c960f..e0c819b68cb 100644
--- a/tests/expression.py
+++ b/tests/expression.py
@@ -149,6 +149,36 @@ async def f():
     signal.getsignal(signal.SIGINT) != signal.default_int_handler
 ):
     return True
+if (
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa |
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+):
+    return True
+if (
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa &
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+):
+    return True
+if (
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa +
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+):
+    return True
+if (
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa -
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+):
+    return True
+if (
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa *
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+):
+    return True
+if (
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa /
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+):
+    return True
 last_call()
 # standalone comment at ENDMARKER
 
@@ -329,5 +359,41 @@ async def f():
 ):
     return True
 
+if (
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+    | aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+):
+    return True
+
+if (
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+    & aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+):
+    return True
+
+if (
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+    + aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+):
+    return True
+
+if (
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+    - aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+):
+    return True
+
+if (
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+    * aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+):
+    return True
+
+if (
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+    / aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+):
+    return True
+
 last_call()
 # standalone comment at ENDMARKER
diff --git a/tests/import_spacing.py b/tests/import_spacing.py
index 87a5301a323..e9f7330db32 100644
--- a/tests/import_spacing.py
+++ b/tests/import_spacing.py
@@ -21,16 +21,16 @@
 from . import tasks
 
 __all__ = (
-    base_events.__all__ +
-    coroutines.__all__ +
-    events.__all__ +
-    futures.__all__ +
-    locks.__all__ +
-    protocols.__all__ +
-    runners.__all__ +
-    queues.__all__ +
-    streams.__all__ +
-    tasks.__all__
+    base_events.__all__
+    + coroutines.__all__
+    + events.__all__
+    + futures.__all__
+    + locks.__all__
+    + protocols.__all__
+    + runners.__all__
+    + queues.__all__
+    + streams.__all__
+    + tasks.__all__
 )
 
 
@@ -60,14 +60,14 @@
 from . import tasks
 
 __all__ = (
-    base_events.__all__ +
-    coroutines.__all__ +
-    events.__all__ +
-    futures.__all__ +
-    locks.__all__ +
-    protocols.__all__ +
-    runners.__all__ +
-    queues.__all__ +
-    streams.__all__ +
-    tasks.__all__
+    base_events.__all__
+    + coroutines.__all__
+    + events.__all__
+    + futures.__all__
+    + locks.__all__
+    + protocols.__all__
+    + runners.__all__
+    + queues.__all__
+    + streams.__all__
+    + tasks.__all__
 )