Relax loop-var-prefix checking

Fixes: #2838

.ansible-lint

@@ -31,7 +31,7 @@ mock_roles:
   - fake_namespace.fake_collection.fake_role # role within a collection
 
 # Enable checking of loop variable prefixes in roles
-loop_var_prefix: "{role}_"
+loop_var_prefix: "^(__|{role}_)"
 
 # Enforce variable names to follow pattern below, in addition to Ansible own
 # requirements, like avoiding python identifiers. To disable add `var-naming`

examples/roles/loop_var_prefix/tasks/pass.yml

@@ -18,3 +18,13 @@
         - bar
       loop_control:
         loop_var: loop_var_prefix_item
+- name: pass | Using alternative double dunder prefix
+  block:
+    - name: pass | That should also pass
+      ansible.builtin.debug:
+        var: __some_item
+      loop:
+        - foo
+        - bar
+      loop_control:
+        loop_var: __some_item

src/ansiblelint/config.py

@@ -91,6 +91,8 @@
 
 PROFILES = yaml_from_file(Path(__file__).parent / "data" / "profiles.yml")
 
+LOOP_VAR_PREFIX = "^(__|{role}_)"
+
 options = Namespace(
     cache_dir=None,
     colored=True,

src/ansiblelint/rules/loop_var_prefix.md

@@ -1,16 +1,33 @@
 # loop-var-prefix
 
-This rule avoids conflicts with nested looping tasks by configuring a variable prefix with `loop_var`.
-Ansible sets `item` as the loop variable.
-You can use `loop_var` to specify a prefix for loop variables and ensure they are unique to each task.
+This rule avoids conflicts with nested looping tasks by configuring a variable
+prefix with `loop_var`. Ansible sets `item` as the loop variable. You can use
+`loop_var` to specify a prefix for loop variables and ensure they are unique to
+each task.
 
 This rule can produce the following messages:
 
-- `loop-var-prefix[missing]` - Replace any unsafe implicit `item` loop variable by adding `loop_var: <loop_var_prefix>...`.
-- `loop-var-prefix[wrong]` - Ensure loop variables start with `<loop_var_prefix>`.
+- `loop-var-prefix[missing]` - Replace any unsafe implicit `item` loop variable
+  by adding `loop_var: <loop_var_prefix>...`.
+- `loop-var-prefix[wrong]` - Ensure loop variables start with
+  `<loop_var_prefix>`.
 
-This is an opt-in rule.
-You must enable it in your Ansible-lint configuration as follows:
+This rule originates from the [Naming parameters section of Ansible Best
+Practices guide][cop314].
+
+## Settings
+
+You can change the behavior of this rule by overriding its default regular
+expression used to check loop variable naming. Keep in mind that the `{role}`
+part is replaced with the inferred role name when applicable.
+
+```yaml
+# .ansible-lint
+loop_var_prefix: "^(__|{role}_)"
+```
+
+This is an opt-in rule. You must enable it in your Ansible-lint configuration as
+follows:
 
 ```yaml
 enable_list:
@@ -56,3 +73,6 @@ enable_list:
       loop_control:
         loop_var: my_prefix # <- Specifies a unique prefix for loop variables.
 ```
+
+[cop314]:
+  https://redhat-cop.github.io/automation-good-practices/#_naming_parameters

src/ansiblelint/rules/loop_var_prefix.py

@@ -1,10 +1,11 @@
 """Optional Ansible-lint rule to enforce use of prefix on role loop vars."""
 from __future__ import annotations
 
+import re
 import sys
 from typing import TYPE_CHECKING, Any
 
-from ansiblelint.config import options
+from ansiblelint.config import LOOP_VAR_PREFIX, options
 from ansiblelint.errors import MatchError
 from ansiblelint.rules import AnsibleLintRule
 from ansiblelint.text import toidentifier
@@ -26,7 +27,7 @@ class RoleLoopVarPrefix(AnsibleLintRule):
 """
 
     tags = ["idiom"]
-    prefix = ""
+    prefix = re.compile("")
     severity = "MEDIUM"
 
     def matchtask(
@@ -36,9 +37,9 @@ def matchtask(
         if not file or not file.role or not options.loop_var_prefix:
             return []
 
-        self.prefix = options.loop_var_prefix.format(role=toidentifier(file.role))
-        # self.prefix becomes `loop_var_prefix_` because role name is loop_var_prefix
-
+        self.prefix = re.compile(
+            options.loop_var_prefix.format(role=toidentifier(file.role))
+        )
         has_loop = "loop" in task
         for key in task.keys():
             if key.startswith("with_"):
@@ -49,18 +50,18 @@ def matchtask(
             loop_var = loop_control.get("loop_var", "")
 
             if loop_var:
-                if not loop_var.startswith(self.prefix):
+                if not self.prefix.match(loop_var):
                     return [
                         self.create_matcherror(
-                            message=f"Loop variable should start with {self.prefix}",
+                            message=f"Loop variable name does not match /{options.loop_var_prefix}/ regex, where role={toidentifier(file.role)}.",
                             filename=file,
                             tag="loop-var-prefix[wrong]",
                         )
                     ]
             else:
                 return [
                     self.create_matcherror(
-                        message=f"Replace unsafe implicit `item` loop variable by adding `loop_var: {self.prefix}...`.",
+                        message=f"Replace unsafe implicit `item` loop variable by adding a `loop_var` that is matching /{options.loop_var_prefix}/ regex.",
                         filename=file,
                         tag="loop-var-prefix[missing]",
                     )
@@ -92,7 +93,7 @@ def test_loop_var_prefix(
     ) -> None:
         """Test rule matches."""
         # Enable checking of loop variable prefixes in roles
-        options.loop_var_prefix = "{role}_"
+        options.loop_var_prefix = LOOP_VAR_PREFIX
         results = Runner(test_file, rules=default_rules_collection).run()
         for result in results:
             assert result.rule.id == RoleLoopVarPrefix().id