From 9394de150ebf0adc426523f46dc08e8b2b2b0b63 Mon Sep 17 00:00:00 2001
From: dylanjblack <38996120+dylanjblack@users.noreply.github.com>
Date: Sat, 15 Jun 2019 14:49:49 +1000
Subject: [PATCH 1/1] Fix trailing comma for function with one arg (#880)
 (#891)

Modified maybe_remove_trailing_comma to remove trailing commas for
typedarglists (in addition to arglists), and updated line split logic
to ensure that all lines in a function definition that contain only one
arg have a trailing comma.
---
 black.py                              | 15 +++++++++++----
 tests/data/function_trailing_comma.py | 14 ++++++++++++++
 tests/test_black.py                   |  8 ++++++++
 3 files changed, 33 insertions(+), 4 deletions(-)
 create mode 100644 tests/data/function_trailing_comma.py

diff --git a/black.py b/black.py
index 635eba2..8318674 100644
--- a/black.py
+++ b/black.py
@@ -1352,7 +1352,10 @@ class Line:
             bracket_depth = leaf.bracket_depth
             if bracket_depth == depth and leaf.type == token.COMMA:
                 commas += 1
-                if leaf.parent and leaf.parent.type == syms.arglist:
+                if leaf.parent and leaf.parent.type in {
+                    syms.arglist,
+                    syms.typedargslist,
+                }:
                     commas += 1
                     break
 
@@ -2488,9 +2491,13 @@ def bracket_split_build_line(
         if leaves:
             # Since body is a new indent level, remove spurious leading whitespace.
             normalize_prefix(leaves[0], inside_brackets=True)
-            # Ensure a trailing comma for imports, but be careful not to add one after
-            # any comments.
-            if original.is_import:
+            # Ensure a trailing comma for imports and standalone function arguments, but
+            # be careful not to add one after any comments.
+            no_commas = original.is_def and not any(
+                l.type == token.COMMA for l in leaves
+            )
+
+            if original.is_import or no_commas:
                 for i in range(len(leaves) - 1, -1, -1):
                     if leaves[i].type == STANDALONE_COMMENT:
                         continue
diff --git a/tests/data/function_trailing_comma.py b/tests/data/function_trailing_comma.py
new file mode 100644
index 0000000..29fd99b
--- /dev/null
+++ b/tests/data/function_trailing_comma.py
@@ -0,0 +1,14 @@
+def f(a,):
+    ...
+
+def f(a:int=1,):
+    ...
+
+# output
+
+def f(a):
+    ...
+
+
+def f(a: int = 1):
+    ...
diff --git a/tests/test_black.py b/tests/test_black.py
index 88c03d0..828b3e4 100644
--- a/tests/test_black.py
+++ b/tests/test_black.py
@@ -264,6 +264,14 @@ class BlackTestCase(unittest.TestCase):
         black.assert_equivalent(source, actual)
         black.assert_stable(source, actual, black.FileMode())
 
+    @patch("black.dump_to_file", dump_to_stderr)
+    def test_function_trailing_comma(self) -> None:
+        source, expected = read_data("function_trailing_comma")
+        actual = fs(source)
+        self.assertFormatEqual(expected, actual)
+        black.assert_equivalent(source, actual)
+        black.assert_stable(source, actual, black.FileMode())
+
     @patch("black.dump_to_file", dump_to_stderr)
     def test_expression(self) -> None:
         source, expected = read_data("expression")
-- 
2.39.5