fix: make python -m tabcmd work reliably (#375)

tabcmd/__main__.py

@@ -2,15 +2,19 @@
 
 try:
     from tabcmd.tabcmd import main
-except ImportError as e:
-    print("Exception thrown running program: `{}`, `{}`".format(e, e.__context__), file=sys.stderr)
-
+except Exception as e:
+    ctx = ", caused by: {}".format(e.__context__) if e.__context__ else ""
+    print("Exception thrown importing tabcmd: {}{}".format(e, ctx), file=sys.stderr)
     if getattr(sys, "frozen", False) and hasattr(sys, "_MEIPASS"):
-        print("Application is running in an executable bundle")
-        print("sys.argv[0] is", sys.argv[0])
-        print("sys.executable is", sys.executable)
+        print("Application is running in an executable bundle", file=sys.stderr)
+        print("sys.argv[0] is", sys.argv[0], file=sys.stderr)
+        print("sys.executable is", sys.executable, file=sys.stderr)
     else:
-        print("[Possible cause: Tabcmd needs to be run as a module, try running `python -m tabcmd`]", file=sys.stderr)
+        print(
+            "[Possible cause: tabcmd is installed but broken, try `pip install --force-reinstall tabcmd`]",
+            file=sys.stderr,
+        )
+    sys.exit(1)
 
 
 if __name__ == "__main__":

tests/test_main.py

@@ -0,0 +1,79 @@
+import os
+import subprocess
+import sys
+import tempfile
+import textwrap
+import unittest
+
+_REPO_ROOT = os.path.abspath(os.path.join(os.path.dirname(__file__), ".."))
+
+
+def _repo_env():
+    env = dict(os.environ)
+    env["PYTHONPATH"] = _REPO_ROOT
+    return env
+
+
+class TestPythonMTabcmd(unittest.TestCase):
+    def test_python_m_tabcmd_no_args_exits_zero(self):
+        result = subprocess.run(
+            [sys.executable, "-m", "tabcmd"],
+            capture_output=True,
+            text=True,
+            timeout=30,
+            env=_repo_env(),
+        )
+        self.assertEqual(result.returncode, 0, msg="stderr: {}\nstdout: {}".format(result.stderr, result.stdout))
+
+    def test_python_m_tabcmd_help_exits_zero(self):
+        result = subprocess.run(
+            [sys.executable, "-m", "tabcmd", "--help"],
+            capture_output=True,
+            text=True,
+            timeout=30,
+            env=_repo_env(),
+        )
+        self.assertEqual(result.returncode, 0, msg="stderr: {}\nstdout: {}".format(result.stderr, result.stdout))
+        self.assertIn("usage:", result.stdout.lower())
+
+    def test_python_m_tabcmd_help_subcommand_exits_zero(self):
+        result = subprocess.run(
+            [sys.executable, "-m", "tabcmd", "help"],
+            capture_output=True,
+            text=True,
+            timeout=30,
+            env=_repo_env(),
+        )
+        self.assertEqual(result.returncode, 0, msg="stderr: {}\nstdout: {}".format(result.stderr, result.stdout))
+
+    def test_import_error_exits_one(self):
+        # Inject a broken tabcmd.tabcmd into sys.modules before running __main__,
+        # so the ImportError handler is exercised without relying on sys.path ordering.
+        script = textwrap.dedent(
+            """\
+            import sys
+            class _BrokenTabcmd:
+                def __getattr__(self, _):
+                    raise ImportError("injected failure")
+            sys.modules["tabcmd.tabcmd"] = _BrokenTabcmd()
+            import runpy
+            runpy.run_module("tabcmd", run_name="__main__")
+            """
+        )
+        with tempfile.TemporaryDirectory() as tmpdir:
+            script_path = os.path.join(tmpdir, "run.py")
+            with open(script_path, "w") as f:
+                f.write(script)
+            result = subprocess.run(
+                [sys.executable, script_path],
+                capture_output=True,
+                text=True,
+                timeout=30,
+                env=_repo_env(),
+            )
+        self.assertEqual(result.returncode, 1, msg="stderr: {}\nstdout: {}".format(result.stderr, result.stdout))
+        self.assertIn("Exception thrown importing tabcmd", result.stderr)
+
+
+if __name__ == "__main__":
+    unittest.main()