Add a test to demonstrate issue 13 is not an issue

src/wxflow/__init__.py

@@ -4,7 +4,7 @@
 from .configuration import (Configuration, cast_as_dtype,
                             cast_strdict_as_dtypedict)
 from .exceptions import WorkflowException, msg_except_handle
-from .executable import CommandNotFoundError, Executable, which
+from .executable import CommandNotFoundError, Executable, ProcessError, which
 from .factory import Factory
 from .file_utils import FileHandler
 from .fsutils import chdir, cp, mkdir, mkdir_p, rm_p, rmdir

src/wxflow/executable.py

@@ -4,7 +4,7 @@
 import sys
 from typing import Any, List, Optional, Union
 
-__all__ = ["Executable", "which", "CommandNotFoundError"]
+__all__ = ["Executable", "which", "CommandNotFoundError", "ProcessError"]
 
 
 class Executable:

tests/test_executable.py

@@ -3,7 +3,7 @@
 
 import pytest
 
-from wxflow import CommandNotFoundError, Executable, which
+from wxflow import CommandNotFoundError, Executable, ProcessError, which
 
 script = """#!/bin/bash
 echo ${USER}
@@ -59,3 +59,31 @@ def test_which(tmpdir):
         exe = which("test.x")
         assert exe is not None
         assert exe.path == path
+
+
+def test_stderr(tmp_path):
+    """
+    Tests the `stderr` attribute of the `Executable` class
+    """
+
+    os.environ["PATH"] = "/usr/bin:/bin"
+
+    cmd = which("ls", required=True)
+
+    stdout_file = tmp_path / 'stdout'
+    stderr_file = tmp_path / 'stderr'
+    cmd("--myopt", output=str(stdout_file), error=str(stderr_file), fail_on_error=False)
+
+    # Assert there is no stdout
+    with open(str(stdout_file)) as fh:
+        assert fh.read() == ''
+
+    # Assert stderr is not empty, '--help' is an unrecognized option
+    with open(str(stderr_file)) as fh:
+        stderr = fh.read()
+        assert stderr != ''
+        print(stderr)
+        # Depending on the OS, the error message may vary
+        # This was seen on macOS
+        # assert stderr == "ls: unrecognized option `--myopt'" + '\n' + \
+        # "usage: ls [-@ABCFGHILOPRSTUWabcdefghiklmnopqrstuvwxy1%,] [--color=when] [-D format] [file ...]" + '\n'