testdir.popen: use kwargs with defaults for stdout/stderr

changelog/5059.trivial.rst

@@ -0,0 +1 @@
+pytester's ``Testdir.popen()`` uses ``stdout`` and ``stderr`` via keyword arguments with defaults now (``subprocess.PIPE``).

src/_pytest/pytester.py

@@ -1034,7 +1034,14 @@ class Testdir(object):
             if colitem.name == name:
                 return colitem
 
-    def popen(self, cmdargs, stdout, stderr, stdin=CLOSE_STDIN, **kw):
+    def popen(
+        self,
+        cmdargs,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+        stdin=CLOSE_STDIN,
+        **kw
+    ):
         """Invoke subprocess.Popen.
 
         This calls subprocess.Popen making sure the current working directory

testing/test_pytester.py

@@ -540,3 +540,22 @@ def test_popen_stdin_bytes(testdir):
     assert stdout.decode("utf8").splitlines() == ["input", "2ndline"]
     assert stderr == b""
     assert proc.returncode == 0
+
+
+def test_popen_default_stdin_stderr_and_stdin_None(testdir):
+    # stdout, stderr default to pipes,
+    # stdin can be None to not close the pipe, avoiding
+    # "ValueError: flush of closed file" with `communicate()`.
+    p1 = testdir.makepyfile(
+        """
+        import sys
+        print(sys.stdin.read())  # empty
+        print('stdout')
+        sys.stderr.write('stderr')
+        """
+    )
+    proc = testdir.popen([sys.executable, str(p1)], stdin=None)
+    stdout, stderr = proc.communicate(b"ignored")
+    assert stdout.splitlines() == [b"", b"stdout"]
+    assert stderr.splitlines() == [b"stderr"]
+    assert proc.returncode == 0