[ledd] Minor refactor; add unit tests (sonic-net#143)

- Refactor ledd:
    - Remove useless try/catch from around imports
    - Move argument parsing out of `DaemonLedd.run()` method and into `main()` function, a more appropriate location
    - Fix LGTM alert for unreachable code

- Add unit tests and report coverage:
    - Test passing good and bad command-line arguments to ledd process

Unit test coverage with this patch:
```
----------- coverage: platform linux, python 3.7.3-final-0 -----------
Name           Stmts   Miss  Cover
----------------------------------
scripts/ledd      66     34    48%
Coverage HTML written to dir htmlcov
Coverage XML written to file coverage.xml
```

sonic-ledd/pytest.ini

@@ -0,0 +1,2 @@
+[pytest]
+addopts = --cov=scripts --cov-report html --cov-report term --cov-report xml

sonic-ledd/scripts/ledd

@@ -5,16 +5,13 @@
     Front-panel LED control daemon for SONiC
 """
 
-try:
-    import getopt
-    import sys
+import getopt
+import sys
 
-    from sonic_py_common import daemon_base
-    from sonic_py_common import multi_asic
-    from sonic_py_common.interface import backplane_prefix
-    from swsscommon import swsscommon
-except ImportError as e:
-    raise ImportError(str(e) + " - required module not found")
+from sonic_py_common import daemon_base
+from sonic_py_common import multi_asic
+from sonic_py_common.interface import backplane_prefix
+from swsscommon import swsscommon
 
 #============================= Constants =============================
 
@@ -42,25 +39,6 @@ class DaemonLedd(daemon_base.DaemonBase):
 
     # Run daemon
     def run(self):
-        # Parse options if provided
-        if (len(sys.argv) > 1):
-            try:
-                (options, remainder) = getopt.getopt(sys.argv[1:],
-                                                     'hv',
-                                                     ['help', 'version'])
-            except getopt.GetoptError as e:
-                print(e)
-                print(USAGE_HELP)
-                sys.exit(2)
-
-            for opt, arg in options:
-                if opt == '--help' or opt == '-h':
-                    print(USAGE_HELP)
-                    sys.exit(0)
-                elif opt == '--version' or opt == '-v':
-                    print('ledd version ' + VERSION)
-                    sys.exit(0)
-
         # Load platform-specific LedControl module
         try:
             led_control = self.load_platform_util(LED_MODULE_NAME, LED_CLASS_NAME)
@@ -117,10 +95,25 @@ class DaemonLedd(daemon_base.DaemonBase):
                     if not key.startswith(backplane_prefix()):
                         led_control.port_link_state_change(key, fvp_dict["oper_status"])
 
-        return 1
-
 
 def main():
+    # Parse options if provided
+    if len(sys.argv) > 1:
+        try:
+            (options, remainder) = getopt.getopt(sys.argv[1:], 'hv', ['help', 'version'])
+        except getopt.GetoptError as e:
+            print(e)
+            print(USAGE_HELP)
+            sys.exit(1)
+
+        for opt, arg in options:
+            if opt == '--help' or opt == '-h':
+                print(USAGE_HELP)
+                sys.exit(0)
+            elif opt == '--version' or opt == '-v':
+                print('ledd version {}'.format(VERSION))
+                sys.exit(0)
+
     ledd = DaemonLedd(SYSLOG_IDENTIFIER)
     ledd.run()
 

sonic-ledd/setup.cfg

@@ -0,0 +1,2 @@
+[aliases]
+test=pytest

sonic-ledd/setup.py

@@ -16,6 +16,11 @@
     setup_requires=[
         'wheel'
     ],
+    tests_require=[
+        'mock>=2.0.0; python_version < "3.3"',
+        'pytest',
+        'pytest-cov'
+    ],
     classifiers=[
         'Development Status :: 4 - Beta',
         'Environment :: No Input/Output (Daemon)',

sonic-ledd/tests/mock_swsscommon.py

@@ -0,0 +1,28 @@
+STATE_DB = ''
+
+
+class Table:
+    def __init__(self, db, table_name):
+        self.table_name = table_name
+        self.mock_dict = {}
+
+    def _del(self, key):
+        del self.mock_dict[key]
+        pass
+
+    def set(self, key, fvs):
+        self.mock_dict[key] = fvs.fv_dict
+        pass
+
+    def get(self, key):
+        if key in self.mock_dict:
+            return self.mock_dict[key]
+        return None
+
+
+class FieldValuePairs(dict):
+    def __init__(self, len):
+        self.fv_dict = {}
+
+    def __setitem__(self, key, val_tuple):
+        self.fv_dict[val_tuple[0]] = val_tuple[1]

sonic-ledd/tests/test_ledd.py

@@ -0,0 +1,68 @@
+import os
+import sys
+from imp import load_source
+
+import pytest
+# TODO: Clean this up once we no longer need to support Python 2
+if sys.version_info.major == 3:
+    from unittest.mock import Mock, MagicMock, patch
+else:
+    from mock import Mock, MagicMock, patch
+from sonic_py_common import daemon_base
+
+SYSLOG_IDENTIFIER = 'ledd_test'
+NOT_AVAILABLE = 'N/A'
+
+daemon_base.db_connect = MagicMock()
+
+test_path = os.path.dirname(os.path.abspath(__file__))
+modules_path = os.path.dirname(test_path)
+scripts_path = os.path.join(modules_path, "scripts")
+sys.path.insert(0, modules_path)
+
+os.environ["LEDD_UNIT_TESTING"] = "1"
+load_source('ledd', scripts_path + '/ledd')
+import ledd
+
+
+def test_help_args(capsys):
+    for flag in ['-h', '--help']:
+        with patch.object(sys, 'argv', ['ledd', flag]):
+            with pytest.raises(SystemExit) as pytest_wrapped_e:
+                ledd.main()
+            assert pytest_wrapped_e.type == SystemExit
+            assert pytest_wrapped_e.value.code == 0
+            out, err = capsys.readouterr()
+            assert out.rstrip() == ledd.USAGE_HELP.rstrip()
+
+
+def test_version_args(capsys):
+    for flag in ['-v', '--version']:
+        with patch.object(sys, 'argv', ['ledd', flag]):
+            with pytest.raises(SystemExit) as pytest_wrapped_e:
+                ledd.main()
+            assert pytest_wrapped_e.type == SystemExit
+            assert pytest_wrapped_e.value.code == 0
+            out, err = capsys.readouterr()
+            assert out.rstrip() == 'ledd version {}'.format(ledd.VERSION)
+
+
+def test_bad_args(capsys):
+    for flag in ['-n', '--nonexistent']:
+        with patch.object(sys, 'argv', ['ledd', flag]):
+            with pytest.raises(SystemExit) as pytest_wrapped_e:
+                ledd.main()
+            assert pytest_wrapped_e.type == SystemExit
+            assert pytest_wrapped_e.value.code == 1
+            out, err = capsys.readouterr()
+            assert out.rstrip().endswith(ledd.USAGE_HELP.rstrip())
+
+
+class TestDaemonLedd(object):
+    """
+    Test cases to cover functionality in DaemonLedd class
+    """
+
+    def test_run(self):
+        daemon_ledd = ledd.DaemonLedd(SYSLOG_IDENTIFIER)
+        # TODO: Add more coverage