Two blank lines after an import should be reduced to one (#4489)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

CHANGES.md

@@ -18,6 +18,7 @@
 <!-- Changes that affect Black's preview style -->
 
 - Remove parentheses around sole list items (#4312)
+- Collapse multiple empty lines after an import into one (#4489)
 
 ### Configuration
 

docs/the_black_code_style/future_style.md

@@ -41,6 +41,8 @@ Currently, the following features are included in the preview style:
 - `remove_lone_list_item_parens`: remove redundant parentheses around lone list items
   (depends on unstable `hug_parens_with_braces_and_square_brackets` feature in some
   cases)
+- `always_one_newline_after_import`: Always force one blank line after import
+  statements, except when the line after the import is a comment or an import statement
 
 (labels/unstable-features)=
 

src/black/lines.py

@@ -671,6 +671,15 @@ def _maybe_empty_lines(self, current_line: Line) -> tuple[int, int]:  # noqa: C9
                 current_line, before, user_had_newline
             )
 
+        if (
+            self.previous_line.is_import
+            and self.previous_line.depth == 0
+            and current_line.depth == 0
+            and not current_line.is_import
+            and Preview.always_one_newline_after_import in self.mode
+        ):
+            return 1, 0
+
         if (
             self.previous_line.is_import
             and not current_line.is_import

src/black/mode.py

@@ -214,6 +214,7 @@ class Preview(Enum):
     # hug_parens_with_braces_and_square_brackets to remove parens in some cases
     remove_lone_list_item_parens = auto()
     pep646_typed_star_arg_type_var_tuple = auto()
+    always_one_newline_after_import = auto()
 
 
 UNSTABLE_FEATURES: set[Preview] = {

src/black/resources/black.schema.json

@@ -92,7 +92,8 @@
           "remove_redundant_guard_parens",
           "parens_for_long_if_clauses_in_case_block",
           "remove_lone_list_item_parens",
-          "pep646_typed_star_arg_type_var_tuple"
+          "pep646_typed_star_arg_type_var_tuple",
+          "always_one_newline_after_import"
         ]
       },
       "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/preview_comments7.py

@@ -177,7 +177,6 @@ def test_fails_invalid_post_data(
     MyLovelyCompanyTeamProjectComponent as component,  # DRY
 )
 
-
 result = 1  # look ma, no comment migration xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
 
 result = 1  # look ma, no comment migration xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

tests/data/cases/preview_import_line_collapse.py

@@ -0,0 +1,180 @@
+# flags: --preview
+from middleman.authentication import validate_oauth_token
+
+
+logger = logging.getLogger(__name__)
+
+
+# case 2 comment after import
+from middleman.authentication import validate_oauth_token
+#comment
+
+logger = logging.getLogger(__name__)
+
+
+# case 3 comment after import
+from middleman.authentication import validate_oauth_token
+# comment
+logger = logging.getLogger(__name__)
+
+
+from middleman.authentication import validate_oauth_token
+
+
+
+logger = logging.getLogger(__name__)
+
+
+# case 4 try catch with import after import
+import os
+import os
+
+
+
+try:
+    import os
+except Exception:
+    pass
+
+try:
+    import os
+    def func():
+        a = 1
+except Exception:
+    pass
+
+
+# case 5 multiple imports
+import os
+import os
+
+import os
+import os
+
+
+
+
+
+for i in range(10):
+    print(i)
+
+
+# case 6 import in function
+def func():
+    print()
+    import os
+    def func():
+        pass
+    print()
+
+
+def func():
+    import os
+    a = 1
+    print()
+
+
+def func():
+    import os
+
+
+    a = 1
+    print()
+
+
+def func():
+    import os
+
+
+
+    a = 1
+    print()
+
+# output
+
+
+from middleman.authentication import validate_oauth_token
+
+logger = logging.getLogger(__name__)
+
+
+# case 2 comment after import
+from middleman.authentication import validate_oauth_token
+
+# comment
+
+logger = logging.getLogger(__name__)
+
+
+# case 3 comment after import
+from middleman.authentication import validate_oauth_token
+
+# comment
+logger = logging.getLogger(__name__)
+
+
+from middleman.authentication import validate_oauth_token
+
+logger = logging.getLogger(__name__)
+
+
+# case 4 try catch with import after import
+import os
+import os
+
+try:
+    import os
+except Exception:
+    pass
+
+try:
+    import os
+
+    def func():
+        a = 1
+
+except Exception:
+    pass
+
+
+# case 5 multiple imports
+import os
+import os
+
+import os
+import os
+
+for i in range(10):
+    print(i)
+
+
+# case 6 import in function
+def func():
+    print()
+    import os
+
+    def func():
+        pass
+
+    print()
+
+
+def func():
+    import os
+
+    a = 1
+    print()
+
+
+def func():
+    import os
+
+    a = 1
+    print()
+
+
+def func():
+    import os
+
+    a = 1
+    print()