Fix check_untyped_defs errors in capture

src/_pytest/capture.py

@@ -12,6 +12,7 @@ from tempfile import TemporaryFile
 
 import pytest
 from _pytest.compat import CaptureIO
+from _pytest.fixtures import FixtureRequest
 
 patchsysdict = {0: "stdin", 1: "stdout", 2: "stderr"}
 
@@ -241,13 +242,12 @@ class CaptureManager:
 capture_fixtures = {"capfd", "capfdbinary", "capsys", "capsysbinary"}
 
 
-def _ensure_only_one_capture_fixture(request, name):
+def _ensure_only_one_capture_fixture(request: FixtureRequest, name):
-    fixtures = set(request.fixturenames) & capture_fixtures - {name}
+    fixtures = sorted(set(request.fixturenames) & capture_fixtures - {name})
     if fixtures:
-        fixtures = sorted(fixtures)
+        arg = fixtures[0] if len(fixtures) == 1 else fixtures
-        fixtures = fixtures[0] if len(fixtures) == 1 else fixtures
         raise request.raiseerror(
-            "cannot use {} and {} at the same time".format(fixtures, name)
+            "cannot use {} and {} at the same time".format(arg, name)
         )
 
 

testing/test_capture.py

@@ -6,6 +6,8 @@ import subprocess
 import sys
 import textwrap
 from io import UnsupportedOperation
+from typing import List
+from typing import TextIO
 
 import py
 
@@ -857,8 +859,8 @@ def tmpfile(testdir):
 
 
 @needsosdup
-def test_dupfile(tmpfile):
+def test_dupfile(tmpfile) -> None:
-    flist = []
+    flist = []  # type: List[TextIO]
     for i in range(5):
         nf = capture.safe_text_dupfile(tmpfile, "wb")
         assert nf != tmpfile