Allow empty line after block open before a comment or compound statement

CHANGES.md

@@ -13,6 +13,7 @@
 ### Preview style
 
 - Fix merging implicit multiline strings that have inline comments (#3956)
+- Allow empty first line after block open before a comment or compound statement (#3967)
 
 ### Configuration
 

src/black/lines.py

@@ -24,6 +24,8 @@
     STANDALONE_COMMENT,
     TEST_DESCENDANTS,
     child_towards,
+    is_docstring,
+    is_funcdef,
     is_import,
     is_multiline_string,
     is_one_sequence_between,
@@ -686,7 +688,30 @@ def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
                 return 0, 1
             return before, 1
 
-        if self.previous_line and self.previous_line.opens_block:
+        is_empty_first_line_ok = (
+            Preview.allow_empty_first_line_before_new_block_or_comment
+            in current_line.mode
+            and (
+                # If it's a standalone comment
+                current_line.leaves[0].type == STANDALONE_COMMENT
+                # If it opens a new block
+                or current_line.opens_block
+                # If it's a triple quote comment (but not at the start of a funcdef)
+                or (
+                    is_docstring(current_line.leaves[0])
+                    and self.previous_line
+                    and self.previous_line.leaves[0]
+                    and self.previous_line.leaves[0].parent
+                    and not is_funcdef(self.previous_line.leaves[0].parent)
+                )
+            )
+        )
+
+        if (
+            self.previous_line
+            and self.previous_line.opens_block
+            and not is_empty_first_line_ok
+        ):
             return 0, 0
         return before, 0
 

src/black/mode.py

@@ -190,6 +190,7 @@ class Preview(Enum):
     module_docstring_newlines = auto()
     accept_raw_docstrings = auto()
     fix_power_op_line_length = auto()
+    allow_empty_first_line_before_new_block_or_comment = auto()
 
 
 class Deprecated(UserWarning):

src/black/nodes.py

@@ -718,6 +718,10 @@ def is_multiline_string(leaf: Leaf) -> bool:
     return has_triple_quotes(leaf.value) and "\n" in leaf.value
 
 
+def is_funcdef(node: Node) -> bool:
+    return node.type == syms.funcdef
+
+
 def is_stub_suite(node: Node) -> bool:
     """Return True if `node` is a suite with a stub body."""
 

tests/data/cases/preview_allow_empty_first_line_in_special_cases.py

@@ -0,0 +1,106 @@
+# flags: --preview
+def foo():
+    """
+    Docstring
+    """
+
+    # Here we go
+    if x:
+
+        # This is also now fine
+        a = 123
+
+    else:
+        # But not necessary
+        a = 123
+
+    if y:
+
+        while True:
+
+            """
+            Long comment here
+            """
+            a = 123
+
+    if z:
+
+        for _ in range(100):
+            a = 123
+    else:
+
+        try:
+
+            # this should be ok
+            a = 123
+        except:
+
+            """also this"""
+            a = 123
+
+
+def bar():
+
+    if x:
+        a = 123
+
+
+def baz():
+
+    # OK
+    if x:
+        a = 123
+
+# output
+
+def foo():
+    """
+    Docstring
+    """
+
+    # Here we go
+    if x:
+
+        # This is also now fine
+        a = 123
+
+    else:
+        # But not necessary
+        a = 123
+
+    if y:
+
+        while True:
+
+            """
+            Long comment here
+            """
+            a = 123
+
+    if z:
+
+        for _ in range(100):
+            a = 123
+    else:
+
+        try:
+
+            # this should be ok
+            a = 123
+        except:
+
+            """also this"""
+            a = 123
+
+
+def bar():
+
+    if x:
+        a = 123
+
+
+def baz():
+
+    # OK
+    if x:
+        a = 123