Fix #4653 If guard explosion in case statement (#4884)

* Updated changelog to resolve conflict

* Gating change to preview style, fixed merge conflict

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* removed extra entries

* added test case

* Fixed preview style and unintentional formatting changes

* removed extra line in mode.py

* added unstable change

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Apply suggestion from @cobaltt7

* Move docs to preview style

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Reorder enum values in black.schema.json

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: cobalt <61329810+cobaltt7@users.noreply.github.com>

Author: 3 people

CHANGES.md

@@ -20,6 +20,8 @@
 
 <!-- Changes that affect Black's preview style -->
 
+- Fix bug where `if` guards in `case` blocks were incorrectly split when the pattern had
+  a trailing comma (#4884)
 - Fix `string_processing` crashing on unassigned long string literals with trailing
   commas (one-item tuples) (#4929)
 - Simplify implementation of the power operator "hugging" logic (#4918)

docs/the_black_code_style/future_style.md

@@ -22,6 +22,9 @@ Currently, the following features are included in the preview style:
   ([see below](labels/simplify-power-operator))
 - `wrap_long_dict_values_in_parens`: Add parentheses around long values in dictionaries.
   ([see below](labels/wrap-long-dict-values))
+- `fix_if_guard_explosion_in_case_statement`: fixed exploding of the if guard in case
+  patterns which have trailing commas in them, even if the guard expression fits in one
+  line
 
 (labels/wrap-comprehension-in)=
 

src/black/linegen.py

@@ -1598,7 +1598,18 @@ def normalize_invisible_parens(
                 break
 
             elif not is_multiline_string(child):
-                wrap_in_parentheses(node, child, visible=False)
+                if (
+                    Preview.fix_if_guard_explosion_in_case_statement in mode
+                    and node.type == syms.guard
+                ):
+                    mock_line = Line(mode=mode)
+                    for leaf in child.leaves():
+                        mock_line.append(leaf)
+                    # If it's a guard AND it's short, we DON'T wrap
+                    if not is_line_short_enough(mock_line, mode=mode):
+                        wrap_in_parentheses(node, child, visible=False)
+                else:
+                    wrap_in_parentheses(node, child, visible=False)
 
         comma_check = child.type == token.COMMA
 

src/black/mode.py

@@ -229,6 +229,7 @@ class Preview(Enum):
     wrap_comprehension_in = auto()
     simplify_power_operator_hugging = auto()
     wrap_long_dict_values_in_parens = auto()
+    fix_if_guard_explosion_in_case_statement = auto()
 
 
 UNSTABLE_FEATURES: set[Preview] = {

src/black/resources/black.schema.json

@@ -84,7 +84,8 @@
           "hug_parens_with_braces_and_square_brackets",
           "wrap_comprehension_in",
           "simplify_power_operator_hugging",
-          "wrap_long_dict_values_in_parens"
+          "wrap_long_dict_values_in_parens",
+          "fix_if_guard_explosion_in_case_statement"
         ]
       },
       "description": "Enable specific features included in the `--unstable` style. Requires `--preview`. No compatibility guarantees are provided on the behavior or existence of any unstable features."

tests/data/cases/if_guard_inside_case.py

@@ -0,0 +1,49 @@
+# flags: --preview
+
+def f(x):
+    match x:
+        # good refactor
+        case [
+            y
+        ] if y == 123:
+            pass
+
+        case [
+            y
+        ] if True:
+            pass
+
+        case [
+            y,
+        ] if True:
+            pass
+
+        # bad refactor
+        case [
+            y,
+        ] if y == 123:
+            pass
+
+
+# output
+
+
+def f(x):
+    match x:
+        # good refactor
+        case [y] if y == 123:
+            pass
+
+        case [y] if True:
+            pass
+
+        case [
+            y,
+        ] if True:
+            pass
+
+        # bad refactor
+        case [
+            y,
+        ] if y == 123:
+            pass