From 35e8d1560d68e8113ff926a9f832582f8f4a694f Mon Sep 17 00:00:00 2001
From: Bryan Forbes <bryan@reigndropsfall.net>
Date: Sun, 2 May 2021 07:48:54 -0500
Subject: [PATCH 1/1] Set `is_pyi` if `stdin_filename` ends with `.pyi` (#2169)

Fixes #2167

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
---
 CHANGES.md            |  6 ++++++
 src/black/__init__.py |  2 ++
 tests/test_black.py   | 32 +++++++++++++++++++++++++++++---
 3 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/CHANGES.md b/CHANGES.md
index 6675c48..321f8c0 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,5 +1,11 @@
 ## Change Log
 
+### Unreleased
+
+#### _Black_
+
+- Set `--pyi` mode if `--stdin-filename` ends in `.pyi` (#2169)
+
 ### 21.4b2
 
 #### _Black_
diff --git a/src/black/__init__.py b/src/black/__init__.py
index 1c69cc4..49d088b 100644
--- a/src/black/__init__.py
+++ b/src/black/__init__.py
@@ -755,6 +755,8 @@ def reformat_one(
             is_stdin = False
 
         if is_stdin:
+            if src.suffix == ".pyi":
+                mode = replace(mode, is_pyi=True)
             if format_stdin_to_stdout(fast=fast, write_back=write_back, mode=mode):
                 changed = Changed.YES
         else:
diff --git a/tests/test_black.py b/tests/test_black.py
index c643f27..43368d4 100644
--- a/tests/test_black.py
+++ b/tests/test_black.py
@@ -1578,8 +1578,34 @@ class BlackTestCase(BlackBaseTestCase):
                 mode=DEFAULT_MODE,
                 report=report,
             )
-            fsts.assert_called_once()
-            # __BLACK_STDIN_FILENAME__ should have been striped
+            fsts.assert_called_once_with(
+                fast=True, write_back=black.WriteBack.YES, mode=DEFAULT_MODE
+            )
+            # __BLACK_STDIN_FILENAME__ should have been stripped
+            report.done.assert_called_with(expected, black.Changed.YES)
+
+    def test_reformat_one_with_stdin_filename_pyi(self) -> None:
+        with patch(
+            "black.format_stdin_to_stdout",
+            return_value=lambda *args, **kwargs: black.Changed.YES,
+        ) as fsts:
+            report = MagicMock()
+            p = "foo.pyi"
+            path = Path(f"__BLACK_STDIN_FILENAME__{p}")
+            expected = Path(p)
+            black.reformat_one(
+                path,
+                fast=True,
+                write_back=black.WriteBack.YES,
+                mode=DEFAULT_MODE,
+                report=report,
+            )
+            fsts.assert_called_once_with(
+                fast=True,
+                write_back=black.WriteBack.YES,
+                mode=replace(DEFAULT_MODE, is_pyi=True),
+            )
+            # __BLACK_STDIN_FILENAME__ should have been stripped
             report.done.assert_called_with(expected, black.Changed.YES)
 
     def test_reformat_one_with_stdin_and_existing_path(self) -> None:
@@ -1603,7 +1629,7 @@ class BlackTestCase(BlackBaseTestCase):
                 report=report,
             )
             fsts.assert_called_once()
-            # __BLACK_STDIN_FILENAME__ should have been striped
+            # __BLACK_STDIN_FILENAME__ should have been stripped
             report.done.assert_called_with(expected, black.Changed.YES)
 
     def test_gitignore_exclude(self) -> None:
-- 
2.39.5