Add `iterating-modified-list` checker for modified lists (#5628)

1 files changed, 154 insertions, 0 deletions

diff --git a/pylint/checkers/modified_iterating_checker.py b/pylint/checkers/modified_iterating_checker.py
new file mode 100644
index 000000000..711d37bb0
--- /dev/null
+++ b/pylint/checkers/modified_iterating_checker.py
@@ -0,0 +1,154 @@
+# Licensed under the GPL: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
+# For details: https://github.com/PyCQA/pylint/blob/main/LICENSE
+
+from typing import TYPE_CHECKING, Union
+
+from astroid import nodes
+
+from pylint import checkers, interfaces
+from pylint.checkers import utils
+
+if TYPE_CHECKING:
+    from pylint.lint import PyLinter
+
+
+_LIST_MODIFIER_METHODS = {"append", "remove"}
+_SET_MODIFIER_METHODS = {"add", "remove"}
+
+
+class ModifiedIterationChecker(checkers.BaseChecker):
+    """Checks for modified iterators in for loops iterations.
+
+    Currently supports `for` loops for Sets, Dictionaries and Lists.
+    """
+
+    __implements__ = interfaces.IAstroidChecker
+
+    name = "modified_iteration"
+
+    msgs = {
+        "W4701": (
+            "Iterated list '%s' is being modified inside for loop body, consider iterating through a copy of it "
+            "instead.",
+            "modified-iterating-list",
+            "Emitted when items are added or removed to a list being iterated through. "
+            "Doing so can result in unexpected behaviour, that is why it is preferred to use a copy of the list.",
+        ),
+        "E4702": (
+            "Iterated dict '%s' is being modified inside for loop body, iterate through a copy of it instead.",
+            "modified-iterating-dict",
+            "Emitted when items are added or removed to a dict being iterated through. "
+            "Doing so raises a RuntimeError.",
+        ),
+        "E4703": (
+            "Iterated set '%s' is being modified inside for loop body, iterate through a copy of it instead.",
+            "modified-iterating-set",
+            "Emitted when items are added or removed to a set being iterated through. "
+            "Doing so raises a RuntimeError.",
+        ),
+    }
+
+    options = ()
+    priority = -2
+
+    @utils.check_messages(
+        "modified-iterating-list", "modified-iterating-dict", "modified-iterating-set"
+    )
+    def visit_for(self, node: nodes.For) -> None:
+        iter_obj = node.iter
+        for body_node in node.body:
+            self._modified_iterating_check_on_node_and_children(body_node, iter_obj)
+
+    def _modified_iterating_check_on_node_and_children(
+        self, body_node: nodes.NodeNG, iter_obj: nodes.NodeNG
+    ) -> None:
+        """See if node or any of its children raises modified iterating messages."""
+        self._modified_iterating_check(body_node, iter_obj)
+        for child in body_node.get_children():
+            self._modified_iterating_check_on_node_and_children(child, iter_obj)
+
+    def _modified_iterating_check(
+        self, node: nodes.NodeNG, iter_obj: nodes.NodeNG
+    ) -> None:
+        msg_id = None
+        if self._modified_iterating_list_cond(node, iter_obj):
+            msg_id = "modified-iterating-list"
+        elif self._modified_iterating_dict_cond(node, iter_obj):
+            msg_id = "modified-iterating-dict"
+        elif self._modified_iterating_set_cond(node, iter_obj):
+            msg_id = "modified-iterating-set"
+        if msg_id:
+            self.add_message(
+                msg_id,
+                node=node,
+                args=(iter_obj.name,),
+                confidence=interfaces.INFERENCE,
+            )
+
+    @staticmethod
+    def _is_node_expr_that_calls_attribute_name(node: nodes.NodeNG) -> bool:
+        return (
+            isinstance(node, nodes.Expr)
+            and isinstance(node.value, nodes.Call)
+            and isinstance(node.value.func, nodes.Attribute)
+            and isinstance(node.value.func.expr, nodes.Name)
+        )
+
+    @staticmethod
+    def _common_cond_list_set(
+        node: nodes.Expr,
+        iter_obj: nodes.NodeNG,
+        infer_val: Union[nodes.List, nodes.Set],
+    ) -> bool:
+        return (infer_val == utils.safe_infer(iter_obj)) and (
+            node.value.func.expr.name == iter_obj.name
+        )
+
+    @staticmethod
+    def _is_node_assigns_subscript_name(node: nodes.NodeNG) -> bool:
+        return isinstance(node, nodes.Assign) and (
+            isinstance(node.targets[0], nodes.Subscript)
+            and (isinstance(node.targets[0].value, nodes.Name))
+        )
+
+    def _modified_iterating_list_cond(
+        self, node: nodes.NodeNG, iter_obj: nodes.NodeNG
+    ) -> bool:
+        if not self._is_node_expr_that_calls_attribute_name(node):
+            return False
+        infer_val = utils.safe_infer(node.value.func.expr)
+        if not isinstance(infer_val, nodes.List):
+            return False
+        return (
+            self._common_cond_list_set(node, iter_obj, infer_val)
+            and node.value.func.attrname in _LIST_MODIFIER_METHODS
+        )
+
+    def _modified_iterating_dict_cond(
+        self, node: nodes.NodeNG, iter_obj: nodes.NodeNG
+    ) -> bool:
+        if not self._is_node_assigns_subscript_name(node):
+            return False
+        infer_val = utils.safe_infer(node.targets[0].value)
+        if not isinstance(infer_val, nodes.Dict):
+            return False
+        if infer_val != utils.safe_infer(iter_obj):
+            return False
+        return node.targets[0].value.name == iter_obj.name
+
+    def _modified_iterating_set_cond(
+        self, node: nodes.NodeNG, iter_obj: nodes.NodeNG
+    ) -> bool:
+        if not self._is_node_expr_that_calls_attribute_name(node):
+            return False
+        infer_val = utils.safe_infer(node.value.func.expr)
+        if not isinstance(infer_val, nodes.Set):
+            return False
+        return (
+            self._common_cond_list_set(node, iter_obj, infer_val)
+            and node.value.func.attrname in _SET_MODIFIER_METHODS
+        )
+
+
+def register(linter: "PyLinter") -> None:
+    linter.register_checker(ModifiedIterationChecker(linter))