From 15bae9fb1ed6f3f0f862ad4322ece56e94dcc48e Mon Sep 17 00:00:00 2001
From: Claudiu Popa <pcmanticore@gmail.com>
Date: Fri, 11 Oct 2019 11:33:33 +0200
Subject: [PATCH] Do not emit ``no-method-argument`` for functions using
 positional only args.

Close #3161
---
 ChangeLog                                     | 4 ++++
 pylint/checkers/classes.py                    | 9 +++++++--
 tests/functional/n/no_method_argument_py38.py | 6 ++++++
 tests/functional/n/no_method_argument_py38.rc | 2 ++
 4 files changed, 19 insertions(+), 2 deletions(-)
 create mode 100644 tests/functional/n/no_method_argument_py38.py
 create mode 100644 tests/functional/n/no_method_argument_py38.rc

diff --git a/ChangeLog b/ChangeLog
index a22e2ce6f9..0a2bd3f673 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -26,6 +26,10 @@ Release date: TBA
 
   Close #3175
 
+* Do not emit ``no-method-argument`` for functions using positional only args.
+
+  Close #3161
+
 
 What's New in Pylint 2.4.2?
 ===========================
diff --git a/pylint/checkers/classes.py b/pylint/checkers/classes.py
index 986809fd34..9f5d099438 100644
--- a/pylint/checkers/classes.py
+++ b/pylint/checkers/classes.py
@@ -1460,7 +1460,12 @@ def _check_first_arg_for_type(self, node, metaclass=0):
         # don't care about functions with unknown argument (builtins)
         if node.args.args is None:
             return
-        first_arg = node.args.args and node.argnames()[0]
+        if node.args.args:
+            first_arg = node.argnames()[0]
+        elif node.args.posonlyargs:
+            first_arg = node.args.posonlyargs[0].name
+        else:
+            first_arg = None
         self._first_attrs.append(first_arg)
         first = self._first_attrs[-1]
         # static method
@@ -1474,7 +1479,7 @@ def _check_first_arg_for_type(self, node, metaclass=0):
                 return
             self._first_attrs[-1] = None
         # class / regular method with no args
-        elif not node.args.args:
+        elif not node.args.args and not node.args.posonlyargs:
             self.add_message("no-method-argument", node=node)
         # metaclass
         elif metaclass:
diff --git a/tests/functional/n/no_method_argument_py38.py b/tests/functional/n/no_method_argument_py38.py
new file mode 100644
index 0000000000..e7a4ded2bd
--- /dev/null
+++ b/tests/functional/n/no_method_argument_py38.py
@@ -0,0 +1,6 @@
+# pylint: disable=missing-docstring
+
+
+class Cls:
+    def __init__(self, obj, /):
+        self.obj = obj
diff --git a/tests/functional/n/no_method_argument_py38.rc b/tests/functional/n/no_method_argument_py38.rc
new file mode 100644
index 0000000000..85fc502b37
--- /dev/null
+++ b/tests/functional/n/no_method_argument_py38.rc
@@ -0,0 +1,2 @@
+[testoptions]
+min_pyver=3.8