Avoid crash on custom rules without doc string (#96)

* Avoid crash on custom rules without doc string

* Add custom rule doc string to docs

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Handle edge case first

Co-authored-by: Aleksey <allburov@gmail.com>

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Aleksey <allburov@gmail.com>

README.md

@@ -388,7 +388,11 @@ from artifactory_cleanup.rules import Rule, ArtifactsList
 
 
 class MySimpleRule(Rule):
-    """For more methods look at Rule source code"""
+    """
+    This doc string is used as rule title
+
+    For more methods look at Rule source code
+    """
 
     def __init__(self, my_param: str, value: int):
         self.my_param = my_param

artifactory_cleanup/rules/base.py

@@ -91,8 +91,9 @@ def name(cls) -> str:
     @classmethod
     def title(cls) -> str:
         """Cut the docstring to show only the very first important line"""
-        docs = [x.strip() for x in cls.__doc__.splitlines() if x][0]
-        return docs
+        if not cls.__doc__:
+            return ""
+        return [x.strip() for x in cls.__doc__.splitlines() if x][0]
 
     def init(self, session, today, *args, **kwargs) -> None:
         """

tests/test_custom_rules.py

@@ -0,0 +1,19 @@
+from artifactory_cleanup.rules import Rule
+
+
+class CustomRuleWithDocs(Rule):
+    """Example with doc string"""
+
+
+class CustomRuleWithoutDocs(Rule):
+    pass
+
+
+def test_custom_rule_with_docstring_adds_to_title():
+    rule = CustomRuleWithDocs()
+    assert rule.title() == "Example with doc string"
+
+
+def test_custom_rule_without_docstring():
+    rule = CustomRuleWithoutDocs()
+    assert rule.title() == ""