Hug power operators if its operands are "simple"

CHANGES.md

@@ -20,6 +20,7 @@
 - Tuple unpacking on `return` and `yield` constructs now implies 3.8+ (#2700)
 - Unparenthesized tuples on annotated assignments (e.g
   `values: Tuple[int, ...] = 1, 2, 3`) now implies 3.8+ (#2708)
+- Remove spaces around power operators if both operands are simple (#2726)
 - Allow setting custom cache directory on all platforms with environment variable
   `BLACK_CACHE_DIR` (#2739).
 - Text coloring added in the final statistics (#2712)

docs/the_black_code_style/current_style.md

@@ -284,6 +284,26 @@ multiple lines. This is so that _Black_ is compliant with the recent changes in
 [PEP 8](https://www.python.org/dev/peps/pep-0008/#should-a-line-break-before-or-after-a-binary-operator)
 style guide, which emphasizes that this approach improves readability.
 
+Almost all operators will be surrounded by single spaces, the only exceptions are unary
+operators (`+`, `-`, and `~`), and power operators when both operands are simple. For
+powers, an operand is considered simple if it's only a NAME, numeric CONSTANT, or
+attribute access (chained attribute access is allowed), with or without a preceding
+unary operator.
+
+```python
+# For example, these won't be surrounded by whitespace
+a = x**y
+b = config.base**5.2
+c = config.base**runtime.config.exponent
+d = 2**5
+e = 2**~5
+
+# ... but these will be surrounded by whitespace
+f = 2 ** get_exponent()
+g = get_x() ** get_y()
+h = config['base'] ** 2
+```
+
 ### Slices
 
 PEP 8

src/black/linegen.py

@@ -21,8 +21,8 @@
 from black.numerics import normalize_numeric_literal
 from black.strings import get_string_prefix, fix_docstring
 from black.strings import normalize_string_prefix, normalize_string_quotes
-from black.trans import Transformer, CannotTransform, StringMerger
-from black.trans import StringSplitter, StringParenWrapper, StringParenStripper
+from black.trans import Transformer, CannotTransform, StringMerger, StringSplitter
+from black.trans import StringParenWrapper, StringParenStripper, hug_power_op
 from black.mode import Mode, Feature, Preview
 
 from blib2to3.pytree import Node, Leaf
@@ -404,6 +404,9 @@ def _rhs(
                 transformers = [delimiter_split, standalone_comment_split, rhs]
             else:
                 transformers = [rhs]
+    # It's always safe to attempt hugging of power operations and pretty much every line
+    # could match.
+    transformers.append(hug_power_op)
 
     for transform in transformers:
         # We are accumulating lines in `result` because we might want to abort

src/black/trans.py

@@ -24,9 +24,9 @@
 import sys
 
 if sys.version_info < (3, 8):
-    from typing_extensions import Final
+    from typing_extensions import Literal, Final
 else:
-    from typing import Final
+    from typing import Literal, Final
 
 from mypy_extensions import trait
 
@@ -71,6 +71,88 @@ def TErr(err_msg: str) -> Err[CannotTransform]:
     return Err(cant_transform)
 
 
+def hug_power_op(line: Line, features: Collection[Feature]) -> Iterator[Line]:
+    """A transformer which normalizes spacing around power operators."""
+
+    # Performance optimization to avoid unnecessary Leaf clones and other ops.
+    for leaf in line.leaves:
+        if leaf.type == token.DOUBLESTAR:
+            break
+    else:
+        raise CannotTransform("No doublestar token was found in the line.")
+
+    def is_simple_lookup(index: int, step: Literal[1, -1]) -> bool:
+        # Brackets and parentheses indicate calls, subscripts, etc. ...
+        # basically stuff that doesn't count as "simple". Only a NAME lookup
+        # or dotted lookup (eg. NAME.NAME) is OK.
+        if step == -1:
+            disallowed = {token.RPAR, token.RSQB}
+        else:
+            disallowed = {token.LPAR, token.LSQB}
+
+        while 0 <= index < len(line.leaves):
+            current = line.leaves[index]
+            if current.type in disallowed:
+                return False
+            if current.type not in {token.NAME, token.DOT} or current.value == "for":
+                # If the current token isn't disallowed, we'll assume this is simple as
+                # only the disallowed tokens are semantically attached to this lookup
+                # expression we're checking. Also, stop early if we hit the 'for' bit
+                # of a comprehension.
+                return True
+
+            index += step
+
+        return True
+
+    def is_simple_operand(index: int, kind: Literal["base", "exponent"]) -> bool:
+        # An operand is considered "simple" if's a NAME, a numeric CONSTANT, a simple
+        # lookup (see above), with or without a preceding unary operator.
+        start = line.leaves[index]
+        if start.type in {token.NAME, token.NUMBER}:
+            return is_simple_lookup(index, step=(1 if kind == "exponent" else -1))
+
+        if start.type in {token.PLUS, token.MINUS, token.TILDE}:
+            if line.leaves[index + 1].type in {token.NAME, token.NUMBER}:
+                # step is always one as bases with a preceding unary op will be checked
+                # for simplicity starting from the next token (so it'll hit the check
+                # above).
+                return is_simple_lookup(index + 1, step=1)
+
+        return False
+
+    leaves: List[Leaf] = []
+    should_hug = False
+    for idx, leaf in enumerate(line.leaves):
+        new_leaf = leaf.clone()
+        if should_hug:
+            new_leaf.prefix = ""
+            should_hug = False
+
+        should_hug = (
+            (0 < idx < len(line.leaves) - 1)
+            and leaf.type == token.DOUBLESTAR
+            and is_simple_operand(idx - 1, kind="base")
+            and line.leaves[idx - 1].value != "lambda"
+            and is_simple_operand(idx + 1, kind="exponent")
+        )
+        if should_hug:
+            new_leaf.prefix = ""
+
+        leaves.append(new_leaf)
+
+    yield Line(
+        mode=line.mode,
+        depth=line.depth,
+        leaves=leaves,
+        comments=line.comments,
+        bracket_tracker=line.bracket_tracker,
+        inside_brackets=line.inside_brackets,
+        should_split_rhs=line.should_split_rhs,
+        magic_trailing_comma=line.magic_trailing_comma,
+    )
+
+
 class StringTransformer(ABC):
     """
     An implementation of the Transformer protocol that relies on its

src/black_primer/primer.json

@@ -81,7 +81,7 @@
     },
     "flake8-bugbear": {
       "cli_arguments": ["--experimental-string-processing"],
-      "expect_formatting_changes": false,
+      "expect_formatting_changes": true,
       "git_clone_url": "https://github.com/PyCQA/flake8-bugbear.git",
       "long_checkout": false,
       "py_versions": ["all"]

tests/data/expression.diff

@@ -11,15 +11,25 @@
  True
  False
  1
-@@ -29,63 +29,96 @@
+@@ -21,71 +21,104 @@
+ Name1 or (Name2 and Name3) or Name4
+ Name1 or Name2 and Name3 or Name4
+ v1 << 2
+ 1 >> v2
+ 1 % finished
+-1 + v2 - v3 * 4 ^ 5 ** v6 / 7 // 8
+-((1 + v2) - (v3 * 4)) ^ (((5 ** v6) / 7) // 8)
++1 + v2 - v3 * 4 ^ 5**v6 / 7 // 8
++((1 + v2) - (v3 * 4)) ^ (((5**v6) / 7) // 8)
+ not great
  ~great
  +value
  -1
  ~int and not v1 ^ 123 + v2 | True
  (~int) and (not ((v1 ^ (123 + v2)) | True))
 -+really ** -confusing ** ~operator ** -precedence
 -flags & ~ select.EPOLLIN and waiters.write_task is not None
-++(really ** -(confusing ** ~(operator ** -precedence)))
+++(really ** -(confusing ** ~(operator**-precedence)))
 +flags & ~select.EPOLLIN and waiters.write_task is not None
  lambda arg: None
  lambda a=True: a
@@ -88,15 +98,19 @@
 +    *more,
 +]
  {i for i in (1, 2, 3)}
- {(i ** 2) for i in (1, 2, 3)}
+-{(i ** 2) for i in (1, 2, 3)}
 -{(i ** 2) for i, _ in ((1, 'a'), (2, 'b'), (3, 'c'))}
-+{(i ** 2) for i, _ in ((1, "a"), (2, "b"), (3, "c"))}
- {((i ** 2) + j) for i in (1, 2, 3) for j in (1, 2, 3)}
+-{((i ** 2) + j) for i in (1, 2, 3) for j in (1, 2, 3)}
++{(i**2) for i in (1, 2, 3)}
++{(i**2) for i, _ in ((1, "a"), (2, "b"), (3, "c"))}
++{((i**2) + j) for i in (1, 2, 3) for j in (1, 2, 3)}
  [i for i in (1, 2, 3)]
- [(i ** 2) for i in (1, 2, 3)]
+-[(i ** 2) for i in (1, 2, 3)]
 -[(i ** 2) for i, _ in ((1, 'a'), (2, 'b'), (3, 'c'))]
-+[(i ** 2) for i, _ in ((1, "a"), (2, "b"), (3, "c"))]
- [((i ** 2) + j) for i in (1, 2, 3) for j in (1, 2, 3)]
+-[((i ** 2) + j) for i in (1, 2, 3) for j in (1, 2, 3)]
++[(i**2) for i in (1, 2, 3)]
++[(i**2) for i, _ in ((1, "a"), (2, "b"), (3, "c"))]
++[((i**2) + j) for i in (1, 2, 3) for j in (1, 2, 3)]
  {i: 0 for i in (1, 2, 3)}
 -{i: j for i, j in ((1, 'a'), (2, 'b'), (3, 'c'))}
 +{i: j for i, j in ((1, "a"), (2, "b"), (3, "c"))}
@@ -181,10 +195,12 @@
  SomeName
  (Good, Bad, Ugly)
  (i for i in (1, 2, 3))
- ((i ** 2) for i in (1, 2, 3))
+-((i ** 2) for i in (1, 2, 3))
 -((i ** 2) for i, _ in ((1, 'a'), (2, 'b'), (3, 'c')))
-+((i ** 2) for i, _ in ((1, "a"), (2, "b"), (3, "c")))
- (((i ** 2) + j) for i in (1, 2, 3) for j in (1, 2, 3))
+-(((i ** 2) + j) for i in (1, 2, 3) for j in (1, 2, 3))
++((i**2) for i in (1, 2, 3))
++((i**2) for i, _ in ((1, "a"), (2, "b"), (3, "c")))
++(((i**2) + j) for i in (1, 2, 3) for j in (1, 2, 3))
  (*starred,)
 -{"id": "1","type": "type","started_at": now(),"ended_at": now() + timedelta(days=10),"priority": 1,"import_session_id": 1,**kwargs}
 +{
@@ -403,13 +419,13 @@
 +    return True
 +if (
 +    ~aaaa.a + aaaa.b - aaaa.c * aaaa.d / aaaa.e
-+    | aaaa.f & aaaa.g % aaaa.h ^ aaaa.i << aaaa.k >> aaaa.l ** aaaa.m // aaaa.n
++    | aaaa.f & aaaa.g % aaaa.h ^ aaaa.i << aaaa.k >> aaaa.l**aaaa.m // aaaa.n
 +):
 +    return True
 +if (
 +    ~aaaaaaaa.a + aaaaaaaa.b - aaaaaaaa.c @ aaaaaaaa.d / aaaaaaaa.e
 +    | aaaaaaaa.f & aaaaaaaa.g % aaaaaaaa.h
-+    ^ aaaaaaaa.i << aaaaaaaa.k >> aaaaaaaa.l ** aaaaaaaa.m // aaaaaaaa.n
++    ^ aaaaaaaa.i << aaaaaaaa.k >> aaaaaaaa.l**aaaaaaaa.m // aaaaaaaa.n
 +):
 +    return True
 +if (
@@ -419,7 +435,7 @@
 +    | aaaaaaaaaaaaaaaa.f & aaaaaaaaaaaaaaaa.g % aaaaaaaaaaaaaaaa.h
 +    ^ aaaaaaaaaaaaaaaa.i
 +    << aaaaaaaaaaaaaaaa.k
-+    >> aaaaaaaaaaaaaaaa.l ** aaaaaaaaaaaaaaaa.m // aaaaaaaaaaaaaaaa.n
++    >> aaaaaaaaaaaaaaaa.l**aaaaaaaaaaaaaaaa.m // aaaaaaaaaaaaaaaa.n
 +):
 +    return True
 +(

tests/data/expression.py

@@ -282,15 +282,15 @@ async def f():
 v1 << 2
 1 >> v2
 1 % finished
-1 + v2 - v3 * 4 ^ 5 ** v6 / 7 // 8
-((1 + v2) - (v3 * 4)) ^ (((5 ** v6) / 7) // 8)
+1 + v2 - v3 * 4 ^ 5**v6 / 7 // 8
+((1 + v2) - (v3 * 4)) ^ (((5**v6) / 7) // 8)
 not great
 ~great
 +value
 -1
 ~int and not v1 ^ 123 + v2 | True
 (~int) and (not ((v1 ^ (123 + v2)) | True))
-+(really ** -(confusing ** ~(operator ** -precedence)))
++(really ** -(confusing ** ~(operator**-precedence)))
 flags & ~select.EPOLLIN and waiters.write_task is not None
 lambda arg: None
 lambda a=True: a
@@ -347,13 +347,13 @@ async def f():
     *more,
 ]
 {i for i in (1, 2, 3)}
-{(i ** 2) for i in (1, 2, 3)}
-{(i ** 2) for i, _ in ((1, "a"), (2, "b"), (3, "c"))}
-{((i ** 2) + j) for i in (1, 2, 3) for j in (1, 2, 3)}
+{(i**2) for i in (1, 2, 3)}
+{(i**2) for i, _ in ((1, "a"), (2, "b"), (3, "c"))}
+{((i**2) + j) for i in (1, 2, 3) for j in (1, 2, 3)}
 [i for i in (1, 2, 3)]
-[(i ** 2) for i in (1, 2, 3)]
-[(i ** 2) for i, _ in ((1, "a"), (2, "b"), (3, "c"))]
-[((i ** 2) + j) for i in (1, 2, 3) for j in (1, 2, 3)]
+[(i**2) for i in (1, 2, 3)]
+[(i**2) for i, _ in ((1, "a"), (2, "b"), (3, "c"))]
+[((i**2) + j) for i in (1, 2, 3) for j in (1, 2, 3)]
 {i: 0 for i in (1, 2, 3)}
 {i: j for i, j in ((1, "a"), (2, "b"), (3, "c"))}
 {a: b * 2 for a, b in dictionary.items()}
@@ -441,9 +441,9 @@ async def f():
 SomeName
 (Good, Bad, Ugly)
 (i for i in (1, 2, 3))
-((i ** 2) for i in (1, 2, 3))
-((i ** 2) for i, _ in ((1, "a"), (2, "b"), (3, "c")))
-(((i ** 2) + j) for i in (1, 2, 3) for j in (1, 2, 3))
+((i**2) for i in (1, 2, 3))
+((i**2) for i, _ in ((1, "a"), (2, "b"), (3, "c")))
+(((i**2) + j) for i in (1, 2, 3) for j in (1, 2, 3))
 (*starred,)
 {
     "id": "1",
@@ -588,13 +588,13 @@ async def f():
     return True
 if (
     ~aaaa.a + aaaa.b - aaaa.c * aaaa.d / aaaa.e
-    | aaaa.f & aaaa.g % aaaa.h ^ aaaa.i << aaaa.k >> aaaa.l ** aaaa.m // aaaa.n
+    | aaaa.f & aaaa.g % aaaa.h ^ aaaa.i << aaaa.k >> aaaa.l**aaaa.m // aaaa.n
 ):
     return True
 if (
     ~aaaaaaaa.a + aaaaaaaa.b - aaaaaaaa.c @ aaaaaaaa.d / aaaaaaaa.e
     | aaaaaaaa.f & aaaaaaaa.g % aaaaaaaa.h
-    ^ aaaaaaaa.i << aaaaaaaa.k >> aaaaaaaa.l ** aaaaaaaa.m // aaaaaaaa.n
+    ^ aaaaaaaa.i << aaaaaaaa.k >> aaaaaaaa.l**aaaaaaaa.m // aaaaaaaa.n
 ):
     return True
 if (
@@ -604,7 +604,7 @@ async def f():
     | aaaaaaaaaaaaaaaa.f & aaaaaaaaaaaaaaaa.g % aaaaaaaaaaaaaaaa.h
     ^ aaaaaaaaaaaaaaaa.i
     << aaaaaaaaaaaaaaaa.k
-    >> aaaaaaaaaaaaaaaa.l ** aaaaaaaaaaaaaaaa.m // aaaaaaaaaaaaaaaa.n
+    >> aaaaaaaaaaaaaaaa.l**aaaaaaaaaaaaaaaa.m // aaaaaaaaaaaaaaaa.n
 ):
     return True
 (