feat: Ignore unused variables starting with _ underscore (#1550)

Author: bhirsz

docs/releasenotes/changelog.md

@@ -2,6 +2,10 @@
 
 ## 7.0.0
 
+### Features
+
+- Ignore unused variables starting with ``_`` (``${_variable}``) ([issue #1457](https://github.com/MarketSquare/robotframework-robocop/issues/1457)
+
 ### Fixes
 
 - Fix ``unused-variable`` and ``variable-overwritten-before-usage`` rules not reporting violations in ``TRY`` blocks ([issue #1548](https://github.com/MarketSquare/robotframework-robocop/issues/1548))

src/robocop/linter/rules/misc.py

@@ -1333,7 +1333,7 @@ def visit_LibraryImport(self, node) -> None:  # noqa: N802
     )
 
     def clear_variables_after_loop(self) -> None:
-        """Remove used variables after loop finishes."""
+        """Remove used variables after the loop finishes."""
         for index, scope in enumerate(self.variables):
             self.variables[index] = {name: variable for name, variable in scope.items() if not variable.is_used}
 
@@ -1474,7 +1474,7 @@ def handle_assign_variable(self, token, ignore_var_conversion: bool = True) -> N
         if ignore_var_conversion:
             name = utils.remove_variable_type_conversion(name)
         normalized = utils.normalize_robot_name(name)
-        if not normalized:  # i.e. "${_}" -> ""
+        if not normalized or name.startswith("_"):  # i.e. "${_}" -> "", or ${_ignore}
             return
         arg = self.arguments.get(normalized, None)
         if arg is not None:

src/robocop/linter/rules/variables.py

@@ -83,18 +83,20 @@ class UnusedVariableRule(Rule):
             Log      Triangle base points are: ${p1} and ${p2}.
             RETURN   ${p1}    ${p2}  # ${p3} is never used
 
-    Use ``${_}`` variable name if you purposefully do not use variable:
+    You can use ``${_}`` variable name or start variable name with ``_`` underscore if you purposefully do not
+    use variable:
 
         *** Keywords ***
         Process Value 10 Times
             [Arguments]    ${value}
             FOR    ${_}   IN RANGE    10
                 Process Value    ${value}
             END
+            ${_first}    ${second}    Unpack List    @{LIST}
 
-    Note that some keywords may use your local variables even if you don't pass them directly. For example
-    BuiltIn ``Replace Variables`` or any custom keyword that retrieves variables from local scope. In such case
-    Robocop will still raise ``unused-variable`` even if variable is used.
+    Note that some keywords may use your local variables even if you don't pass them directly. For example,
+    BuiltIn ``Replace Variables`` or any custom keyword that retrieves variables from a local scope. In this case,
+    Robocop will still raise an ``unused-variable`` even if the variable is actually used.
 
     """
 

tests/linter/rules/variables/unused_variable/test.robot

@@ -255,3 +255,20 @@ Except with AS
     EXCEPT    Error message*  AS  ${var2}
         Log    ${var2}
     END
+
+Not used starting with _
+    [Documentation]    Should be ignored.
+    FOR    ${_var}    IN    1  2  3
+        Keyword
+    END
+    TRY
+        May Fail    ${var1}
+    EXCEPT     Error message*  AS  ${_var2}
+        No Operation
+        ${_branch_assign}    Keyword
+    END
+    IF    condition
+        ${_var3}    Keyword
+    END
+    ${var4}    ${_var5}    Keyword
+    Log    ${var4}