[MO] Fix showing graceful error message when no ngraph case (#6379)

* [MO] Fix showing graceful error message when no ngraph case

Move frontend-related imports inside appropriate functions
And use try/except if import fails to show graceful error message

* Revert "[MO] Fix showing graceful error message when no ngraph case"

This reverts commit 72d52bd946.

* Print error before running of subprocess if IE or nGraph Python API is not found

* Added pytest to requirements_dev.txt

* Fixed review comment

model-optimizer/mo/subprocess_main.py

@@ -1,20 +1,37 @@
 # Copyright (C) 2021 Intel Corporation
 # SPDX-License-Identifier: Apache-2.0
 
+import logging as log
 import os
-import sys
 import subprocess
+import sys
 
 from mo.utils.versions_checker import check_python_version  # pylint: disable=no-name-in-module
 
 
+def log_ie_not_found():
+    log.error("Could not find the Inference Engine or nGraph Python API.\n"
+              "Consider building the Inference Engine and nGraph Python APIs"
+              " from sources or try to install OpenVINO (TM) Toolkit using \"install_prerequisites.{}\""
+              .format("bat" if sys.platform == "windows" else "sh"))
+
+
 def setup_env():
     ret_code = check_python_version()
     if ret_code:
         sys.exit(ret_code)
 
     from mo.utils.find_ie_version import find_ie_version
-    find_ie_version(silent=True)
+
+    ie_found = True
+    try:
+        ie_found = find_ie_version(silent=True)
+    except Exception:
+        ie_found = False
+
+    if not ie_found:
+        log_ie_not_found()
+        sys.exit(1)
 
     mo_root_path = os.path.join(os.path.dirname(__file__), os.pardir)
 
@@ -23,6 +40,7 @@ def setup_env():
         os.environ[python_path_key] = mo_root_path
     else:
         os.environ[python_path_key] = os.pathsep.join([os.environ[python_path_key], mo_root_path])
+    return True
 
 
 def subprocess_main(framework=None):
@@ -45,4 +63,4 @@ def subprocess_main(framework=None):
     for arg in sys.argv[1:]:
         args.append(arg)
     status = subprocess.run(args, env=os.environ)
-    sys.exit(status.returncode)
+    sys.exit(status.returncode)

model-optimizer/requirements_dev.txt

@@ -5,3 +5,4 @@ pyenchant==1.6.11
 test-generator==0.1.1
 defusedxml>=0.5.0
 requests>=2.20.0
+pytest>=6.2.4

model-optimizer/unit_tests/mo/frontend_ngraph_test.py

@@ -1,11 +1,29 @@
 # Copyright (C) 2018-2021 Intel Corporation
 # SPDX-License-Identifier: Apache-2.0
 
+import logging as log
 import os
 import subprocess
 import sys
+import unittest
+from unittest.mock import patch
+
+from mo.subprocess_main import setup_env, subprocess_main
+
+import pytest
+
+
+class TestNoInferenceEngine(unittest.TestCase):
+    @patch('mo.utils.find_ie_version.find_ie_version')
+    def test_no_ie_ngraph(self, mock_find):
+        mock_find.return_value = False
+        with pytest.raises(SystemExit) as e, self.assertLogs(log.getLogger(), level="ERROR") as cm:
+            subprocess_main()
+        assert e.value.code == 1
+        res = [i for i in cm.output if
+               'Consider building the Inference Engine and nGraph Python APIs from sources' in i]
+        assert res
 
-from mo.subprocess_main import setup_env
 
 def test_frontends():
     setup_env()