Add a new check dict-init-mutate (#7794)

Co-authored-by: Pierre Sassoulas <pierre.sassoulas@gmail.com>

doc/data/messages/d/dict-init-mutate/bad.py

@@ -0,0 +1,3 @@
+fruit_prices = {}  # [dict-init-mutate]
+fruit_prices['apple'] = 1
+fruit_prices['banana'] = 10

doc/data/messages/d/dict-init-mutate/good.py

@@ -0,0 +1 @@
+fruit_prices = {"apple": 1, "banana": 10}

doc/data/messages/d/dict-init-mutate/pylintrc

@@ -0,0 +1,2 @@
+[MAIN]
+load-plugins=pylint.extensions.dict_init_mutate,

doc/user_guide/checkers/extensions.rst

@@ -14,6 +14,7 @@ Pylint provides the following optional plugins:
 - :ref:`pylint.extensions.comparison_placement`
 - :ref:`pylint.extensions.confusing_elif`
 - :ref:`pylint.extensions.consider_ternary_expression`
+- :ref:`pylint.extensions.dict_init_mutate`
 - :ref:`pylint.extensions.docparams`
 - :ref:`pylint.extensions.docstyle`
 - :ref:`pylint.extensions.dunder`
@@ -264,6 +265,21 @@ Design checker Messages
   Cyclomatic
 
 
+.. _pylint.extensions.dict_init_mutate:
+
+Dict-Init-Mutate checker
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+This checker is provided by ``pylint.extensions.dict_init_mutate``.
+Verbatim name of the checker is ``dict-init-mutate``.
+
+Dict-Init-Mutate checker Messages
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+:dict-init-mutate (C3401): *Dictionary mutated immediately after initialization*
+  Dictionaries can be initialized with a single statement using dictionary
+  literal syntax.
+
+
 .. _pylint.extensions.docstyle:
 
 Docstyle checker

doc/user_guide/messages/messages_overview.rst

@@ -231,6 +231,7 @@ All messages in the warning category:
    warning/deprecated-method
    warning/deprecated-module
    warning/deprecated-typing-alias
+   warning/dict-init-mutate
    warning/differing-param-doc
    warning/differing-type-doc
    warning/duplicate-except

doc/whatsnew/fragments/2876.new_check

@@ -0,0 +1,4 @@
+Add new extension checker ``dict-init-mutate`` that flags mutating a dictionary immediately
+after the dictionary was created.
+
+Closes #2876

pylint/extensions/dict_init_mutate.py

@@ -0,0 +1,66 @@
+# 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
+# Copyright (c) https://github.com/PyCQA/pylint/blob/main/CONTRIBUTORS.txt
+
+"""Check for use of dictionary mutation after initialization."""
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from astroid import nodes
+
+from pylint.checkers import BaseChecker
+from pylint.checkers.utils import only_required_for_messages
+from pylint.interfaces import HIGH
+
+if TYPE_CHECKING:
+    from pylint.lint.pylinter import PyLinter
+
+
+class DictInitMutateChecker(BaseChecker):
+    name = "dict-init-mutate"
+    msgs = {
+        "C3401": (
+            "Declare all known key/values when initializing the dictionary.",
+            "dict-init-mutate",
+            "Dictionaries can be initialized with a single statement "
+            "using dictionary literal syntax.",
+        )
+    }
+
+    @only_required_for_messages("dict-init-mutate")
+    def visit_assign(self, node: nodes.Assign) -> None:
+        """
+        Detect dictionary mutation immediately after initialization.
+
+        At this time, detecting nested mutation is not supported.
+        """
+        if not isinstance(node.value, nodes.Dict):
+            return
+
+        dict_name = node.targets[0]
+        if len(node.targets) != 1 or not isinstance(dict_name, nodes.AssignName):
+            return
+
+        first_sibling = node.next_sibling()
+        if (
+            not first_sibling
+            or not isinstance(first_sibling, nodes.Assign)
+            or len(first_sibling.targets) != 1
+        ):
+            return
+
+        sibling_target = first_sibling.targets[0]
+        if not isinstance(sibling_target, nodes.Subscript):
+            return
+
+        sibling_name = sibling_target.value
+        if not isinstance(sibling_name, nodes.Name):
+            return
+
+        if sibling_name.name == dict_name.name:
+            self.add_message("dict-init-mutate", node=node, confidence=HIGH)
+
+
+def register(linter: PyLinter) -> None:
+    linter.register_checker(DictInitMutateChecker(linter))

tests/functional/ext/dict_init_mutate.py

@@ -0,0 +1,38 @@
+"""Example cases for dict-init-mutate"""
+# pylint: disable=use-dict-literal, invalid-name
+
+base = {}
+
+fruits = {}
+for fruit in ["apple", "orange"]:
+    fruits[fruit] = 1
+    fruits[fruit] += 1
+
+count = 10
+fruits = {"apple": 1}
+fruits["apple"] += count
+
+config = {}  # [dict-init-mutate]
+config['pwd'] = 'hello'
+
+config = {}  # [dict-init-mutate]
+config['dir'] = 'bin'
+config['user'] = 'me'
+config['workers'] = 5
+print(config)
+
+config = {}  # Not flagging calls to update for now
+config.update({"dir": "bin"})
+
+config = {}  # [dict-init-mutate]
+config['options'] = {}  # Identifying nested assignment not supporting this yet.
+config['options']['debug'] = False
+config['options']['verbose'] = True
+
+
+config = {}
+def update_dict(di):
+    """Update a dictionary"""
+    di["one"] = 1
+
+update_dict(config)

tests/functional/ext/dict_init_mutate.rc

@@ -0,0 +1,2 @@
+[MAIN]
+load-plugins=pylint.extensions.dict_init_mutate,

tests/functional/ext/dict_init_mutate.txt

@@ -0,0 +1,3 @@
+dict-init-mutate:15:0:15:11::Declare all known key/values when initializing the dictionary.:HIGH
+dict-init-mutate:18:0:18:11::Declare all known key/values when initializing the dictionary.:HIGH
+dict-init-mutate:27:0:27:11::Declare all known key/values when initializing the dictionary.:HIGH