util.py: load_yaml: Don't fail when it's not necessary

Also, make 'require()' function not fail by default.
The only case where we really need to fail is when 'kramdown' parser is
not specified. This is a highly unlikely scenario, tbh (because
arguments to `lesson_check.py` are set in the Makefile), but we can
think about reworking/optimizing this part later.

Author: maxim-belkin

bin/lesson_check.py

@@ -180,7 +180,8 @@ def parse_args():
 
     args, extras = parser.parse_known_args()
     require(args.parser is not None,
-            'Path to Markdown parser not provided')
+            'Path to Markdown parser not provided',
+            True)
     require(not extras,
             'Unexpected trailing command-line arguments "{0}"'.format(extras))
 

bin/util.py

@@ -65,9 +65,8 @@ def split_metadata(path, text):
         try:
             metadata_yaml = yaml.load(metadata_raw, Loader=yaml.SafeLoader)
         except yaml.YAMLError as e:
-            print('Unable to parse YAML header in {0}:\n{1}'.format(
-                path, e), file=sys.stderr)
-            sys.exit(1)
+            message = 'Unable to parse YAML header in {0}:\n{1}'
+            print(message.format(path, e), file=sys.stderr)
 
     return metadata_raw, metadata_yaml, text
 
@@ -81,11 +80,14 @@ def load_yaml(filename):
     try:
         with open(filename, 'r', encoding='utf-8') as reader:
             return yaml.load(reader, Loader=yaml.SafeLoader)
-    except (yaml.YAMLError, IOError) as e:
-        print('Unable to load YAML file {0}:\n{1}'.format(
-            filename, e), file=sys.stderr)
-        sys.exit(1)
+    except yaml.YAMLError as e:
+        message = 'ERROR: Unable to load YAML file {0}:\n{1}'
+        print(message.format(filename, e), file=sys.stderr)
+    except (FileNotFoundError, IOError):
+        message = 'ERROR: File {} not found'
+        print(message.format(filename), file=sys.stderr)
 
+    return {}
 
 def check_unwanted_files(dir_path, reporter):
     """
@@ -99,9 +101,11 @@ def check_unwanted_files(dir_path, reporter):
                        "Unwanted file found")
 
 
-def require(condition, message):
+def require(condition, message, fatal=False):
     """Fail if condition not met."""
 
     if not condition:
         print(message, file=sys.stderr)
-        sys.exit(1)
+
+        if fatal:
+            sys.exit(1)