Implement fluent interfaces

Fixes #67

README.md

@@ -342,6 +342,26 @@ In those cases, parentheses are removed when the entire statement fits
 in one line, or if the inner expression doesn't have any delimiters to
 further split on.  Otherwise, the parentheses are always added.
 
+### Call chains
+
+Some popular APIs, like ORMs, use call chaining.  This API style is known
+as a [fluent interface](https://en.wikipedia.org/wiki/Fluent_interface).
+*Black* formats those treating dots that follow a call or an indexing
+operation like a very low priority delimiter.  It's easier to show the
+behavior than to explain it.  Look at the example::
+```py3
+def example(session):
+    result = (
+        session.query(models.Customer.id)
+        .filter(
+            models.Customer.account_id == account_id,
+            models.Customer.email == email_address,
+        )
+        .order_by(models.Customer.id.asc())
+        .all()
+    )
+```
+
 ### Typing stub files
 
 PEP 484 describes the syntax for type hints in Python.  One of the
@@ -589,6 +609,8 @@ More details can be found in [CONTRIBUTING](CONTRIBUTING.md).
 
 ### 18.5a0 (unreleased)
 
+* call chains are now formatted according to the [fluent interfaces](https://en.wikipedia.org/wiki/Fluent_interface) style (#67)
+
 * slices are now formatted according to PEP 8 (#178)
 
 * parentheses are now also managed automatically on the right-hand side

black.py

@@ -626,21 +626,22 @@ def show(cls, code: str) -> None:
 STRING_PRIORITY = 12
 COMPARATOR_PRIORITY = 10
 MATH_PRIORITIES = {
-    token.VBAR: 8,
-    token.CIRCUMFLEX: 7,
-    token.AMPER: 6,
-    token.LEFTSHIFT: 5,
-    token.RIGHTSHIFT: 5,
-    token.PLUS: 4,
-    token.MINUS: 4,
-    token.STAR: 3,
-    token.SLASH: 3,
-    token.DOUBLESLASH: 3,
-    token.PERCENT: 3,
-    token.AT: 3,
-    token.TILDE: 2,
-    token.DOUBLESTAR: 1,
+    token.VBAR: 9,
+    token.CIRCUMFLEX: 8,
+    token.AMPER: 7,
+    token.LEFTSHIFT: 6,
+    token.RIGHTSHIFT: 6,
+    token.PLUS: 5,
+    token.MINUS: 5,
+    token.STAR: 4,
+    token.SLASH: 4,
+    token.DOUBLESLASH: 4,
+    token.PERCENT: 4,
+    token.AT: 4,
+    token.TILDE: 3,
+    token.DOUBLESTAR: 2,
 }
+DOT_PRIORITY = 1
 
 
 @dataclass
@@ -1729,6 +1730,14 @@ def is_split_before_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
         # Don't treat them as a delimiter.
         return 0
 
+    if (
+        leaf.type == token.DOT
+        and leaf.parent
+        and leaf.parent.type not in {syms.import_from, syms.dotted_name}
+        and (previous is None or previous.type != token.NAME)
+    ):
+        return DOT_PRIORITY
+
     if (
         leaf.type in MATH_OPERATORS
         and leaf.parent
@@ -2128,6 +2137,10 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
     except ValueError:
         raise CannotSplit("No delimiters found")
 
+    if delimiter_priority == DOT_PRIORITY:
+        if bt.delimiter_count_with_priority(delimiter_priority) == 1:
+            raise CannotSplit("Splitting a single attribute from its owner looks wrong")
+
     current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
     lowest_depth = sys.maxsize
     trailing_comma_safe = True

tests/comments4.py

@@ -61,28 +61,37 @@ def test_fails_invalid_post_data(
 
 def foo(list_a, list_b):
     results = (
-        User.query.filter(User.foo == "bar").filter(  # Because foo.
+        User.query.filter(User.foo == "bar")
+        .filter(  # Because foo.
             db.or_(User.field_a.astext.in_(list_a), User.field_b.astext.in_(list_b))
-        ).filter(User.xyz.is_(None))
+        )
+        .filter(User.xyz.is_(None))
         # Another comment about the filtering on is_quux goes here.
-        .filter(db.not_(User.is_pending.astext.cast(db.Boolean).is_(True))).order_by(
-            User.created_at.desc()
-        ).with_for_update(key_share=True).all()
+        .filter(db.not_(User.is_pending.astext.cast(db.Boolean).is_(True)))
+        .order_by(User.created_at.desc())
+        .with_for_update(key_share=True)
+        .all()
     )
     return results
 
 
 def foo2(list_a, list_b):
     # Standalone comment reasonably placed.
-    return User.query.filter(User.foo == "bar").filter(
-        db.or_(User.field_a.astext.in_(list_a), User.field_b.astext.in_(list_b))
-    ).filter(User.xyz.is_(None))
+    return (
+        User.query.filter(User.foo == "bar")
+        .filter(
+            db.or_(User.field_a.astext.in_(list_a), User.field_b.astext.in_(list_b))
+        )
+        .filter(User.xyz.is_(None))
+    )
 
 
 def foo3(list_a, list_b):
     return (
         # Standlone comment but weirdly placed.
-        User.query.filter(User.foo == "bar").filter(
+        User.query.filter(User.foo == "bar")
+        .filter(
             db.or_(User.field_a.astext.in_(list_a), User.field_b.astext.in_(list_b))
-        ).filter(User.xyz.is_(None))
+        )
+        .filter(User.xyz.is_(None))
     )

tests/expression.diff

@@ -128,7 +128,7 @@
  ]
  slice[0]
  slice[0:1]
-@@ -124,107 +144,154 @@
+@@ -124,107 +144,159 @@
  numpy[-(c + 1) :, d]
  numpy[:, l[-2]]
  numpy[:, ::-1]
@@ -173,9 +173,14 @@
 +what_is_up_with_those_new_coord_names = (coord_names | set(vars_to_create)) - set(
 +    vars_to_remove
 +)
-+result = session.query(models.Customer.id).filter(
-+    models.Customer.account_id == account_id, models.Customer.email == email_address
-+).order_by(models.Customer.id.asc()).all()
++result = (
++    session.query(models.Customer.id)
++    .filter(
++        models.Customer.account_id == account_id, models.Customer.email == email_address
++    )
++    .order_by(models.Customer.id.asc())
++    .all()
++)
  Ø = set()
  authors.łukasz.say_thanks()
  mapping = {

tests/expression.py

@@ -411,9 +411,14 @@ async def f():
 what_is_up_with_those_new_coord_names = (coord_names | set(vars_to_create)) - set(
     vars_to_remove
 )
-result = session.query(models.Customer.id).filter(
-    models.Customer.account_id == account_id, models.Customer.email == email_address
-).order_by(models.Customer.id.asc()).all()
+result = (
+    session.query(models.Customer.id)
+    .filter(
+        models.Customer.account_id == account_id, models.Customer.email == email_address
+    )
+    .order_by(models.Customer.id.asc())
+    .all()
+)
 Ø = set()
 authors.łukasz.say_thanks()
 mapping = {

tests/function.py

@@ -167,9 +167,15 @@ def spaces2(result=_core.Value(None)):
 
 
 def example(session):
-    result = session.query(models.Customer.id).filter(
-        models.Customer.account_id == account_id, models.Customer.email == email_address
-    ).order_by(models.Customer.id.asc()).all()
+    result = (
+        session.query(models.Customer.id)
+        .filter(
+            models.Customer.account_id == account_id,
+            models.Customer.email == email_address,
+        )
+        .order_by(models.Customer.id.asc())
+        .all()
+    )
 
 
 def long_lines():