Allow specifying stdout and stderr to session.run.

nox/command.py

@@ -69,7 +69,8 @@ def run(
     path=None,
     success_codes=None,
     log=True,
-    external=False
+    external=False,
+    **popen_kws
 ):
     """Run a command-line program."""
 
@@ -105,7 +106,9 @@ def run(
     env = _clean_env(env)
 
     try:
-        return_code, output = popen([cmd_path] + list(args), silent=silent, env=env)
+        return_code, output = popen(
+            [cmd_path] + list(args), silent=silent, env=env, **popen_kws
+        )
 
         if return_code not in success_codes:
             logger.error(

nox/popen.py

@@ -16,13 +16,16 @@
 import sys
 
 
-def popen(args, env=None, silent=False):
-    stdout = None
+def popen(args, env=None, silent=False, stdout=None, stderr=subprocess.STDOUT):
+    if silent and stdout is not None:
+        raise ValueError(
+            "Can not specify silent and stdout; passing a custom stdout always silences the commands output in Nox's log."
+        )
 
     if silent:
         stdout = subprocess.PIPE
 
-    proc = subprocess.Popen(args, env=env, stdout=stdout, stderr=subprocess.STDOUT)
+    proc = subprocess.Popen(args, env=env, stdout=stdout, stderr=stderr)
 
     try:
         out, err = proc.communicate()

tests/test_command.py

@@ -148,3 +148,95 @@ def test_interrupt():
     with mock.patch("subprocess.Popen", return_value=mock_proc):
         with pytest.raises(KeyboardInterrupt):
             nox.command.run([PYTHON, "-c" "123"])
+
+
+def test_custom_stdout(capsys, tmpdir):
+    with open(str(tmpdir / "out.txt"), "w+b") as stdout:
+        nox.command.run(
+            [
+                PYTHON,
+                "-c",
+                'import sys; sys.stdout.write("out");'
+                'sys.stderr.write("err"); sys.exit(0)',
+            ],
+            stdout=stdout,
+        )
+        out, err = capsys.readouterr()
+        assert not out
+        assert "out" not in err
+        assert "err" not in err
+        stdout.seek(0)
+        tempfile_contents = stdout.read().decode("utf-8")
+        assert "out" in tempfile_contents
+        assert "err" in tempfile_contents
+
+
+def test_custom_stdout_silent_flag(capsys, tmpdir):
+    with open(str(tmpdir / "out.txt"), "w+b") as stdout:
+        with pytest.raises(ValueError, match="silent"):
+            nox.command.run([PYTHON, "-c", 'print("hi")'], stdout=stdout, silent=True)
+
+
+def test_custom_stdout_failed_command(capsys, tmpdir):
+    with open(str(tmpdir / "out.txt"), "w+b") as stdout:
+        with pytest.raises(nox.command.CommandFailed):
+            nox.command.run(
+                [
+                    PYTHON,
+                    "-c",
+                    'import sys; sys.stdout.write("out");'
+                    'sys.stderr.write("err"); sys.exit(1)',
+                ],
+                stdout=stdout,
+            )
+        out, err = capsys.readouterr()
+        assert not out
+        assert "out" not in err
+        assert "err" not in err
+        stdout.seek(0)
+        tempfile_contents = stdout.read().decode("utf-8")
+        assert "out" in tempfile_contents
+        assert "err" in tempfile_contents
+
+
+def test_custom_stderr(capsys, tmpdir):
+    with open(str(tmpdir / "err.txt"), "w+b") as stderr:
+        nox.command.run(
+            [
+                PYTHON,
+                "-c",
+                'import sys; sys.stdout.write("out");'
+                'sys.stderr.write("err"); sys.exit(0)',
+            ],
+            stderr=stderr,
+        )
+        out, err = capsys.readouterr()
+        assert not err
+        assert "out" not in out
+        assert "err" not in out
+        stderr.seek(0)
+        tempfile_contents = stderr.read().decode("utf-8")
+        assert "out" not in tempfile_contents
+        assert "err" in tempfile_contents
+
+
+def test_custom_stderr_failed_command(capsys, tmpdir):
+    with open(str(tmpdir / "out.txt"), "w+b") as stderr:
+        with pytest.raises(nox.command.CommandFailed):
+            nox.command.run(
+                [
+                    PYTHON,
+                    "-c",
+                    'import sys; sys.stdout.write("out");'
+                    'sys.stderr.write("err"); sys.exit(1)',
+                ],
+                stderr=stderr,
+            )
+        out, err = capsys.readouterr()
+        assert not err
+        assert "out" not in out
+        assert "err" not in out
+        stderr.seek(0)
+        tempfile_contents = stderr.read().decode("utf-8")
+        assert "out" not in tempfile_contents
+        assert "err" in tempfile_contents