1 files changed, 55 insertions, 0 deletions
diff --git a/pylint/extensions/confusing_elif.py b/pylint/extensions/confusing_elif.py
new file mode 100644
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+++ b/pylint/extensions/confusing_elif.py
@@ -0,0 +1,55 @@
+# Copyright (c) 2021 Andreas Finkler <andi.finkler@gmail.com>
+
+# Licensed under the GPL: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
+# For details: https://github.com/PyCQA/pylint/blob/master/COPYING
+
+import astroid
+
+from pylint.checkers import BaseChecker
+from pylint.checkers.utils import check_messages
+from pylint.interfaces import IAstroidChecker
+from pylint.lint import PyLinter
+
+
+class ConfusingConsecutiveElifChecker(BaseChecker):
+    """Checks if "elif" is used right after an indented block that finishes with "if" or "elif" itself."""
+
+    __implements__ = IAstroidChecker
+
+    name = "confusing-elif-checker"
+    priority = -1
+    msgs = {
+        "R5601": (
+            "Consecutive elif with differing indentation level, consider creating a function to separate the inner elif",
+            "confusing-consecutive-elif",
+            "Used when an elif statement follows right after an indented block which itself ends with if or elif. "
+            "It may not be ovious if the elif statement was willingly or mistakenly unindented. "
+            "Extracting the indented if statement into a separate function might avoid confusion and prevent errors.",
+        )
+    }
+
+    @check_messages("confusing-consecutive-elif")
+    def visit_if(self, node: astroid.If):
+        body_ends_with_if = isinstance(
+            node.body[-1], astroid.If
+        ) and self._has_no_else_clause(node.body[-1])
+        if node.has_elif_block() and body_ends_with_if:
+            self.add_message("confusing-consecutive-elif", node=node.orelse[0])
+
+    @staticmethod
+    def _has_no_else_clause(node: astroid.If):
+        orelse = node.orelse
+        while orelse and isinstance(orelse[0], astroid.If):
+            orelse = orelse[0].orelse
+        if not orelse or isinstance(orelse[0], astroid.If):
+            return True
+        return False
+
+
+def register(linter: PyLinter):
+    """This required method auto registers the checker.
+
+    :param linter: The linter to register the checker to.
+    :type linter: pylint.lint.PyLinter
+    """
+    linter.register_checker(ConfusingConsecutiveElifChecker(linter))