Prevent unused-import for if t.TYPE_CHECKING

doc/whatsnew/2/2.14/full.rst

@@ -22,6 +22,11 @@ Release date: TBA
 
   Closes #6802
 
+* Fixed false positives for ``unused-import`` when aliasing ``typing`` e.g. as ``t``
+  and guarding imports under ``t.TYPE_CHECKING``.
+
+  Closes #3846
+
 * Fixed a false positive regression in 2.13 for ``used-before-assignment`` where it is safe to rely
   on a name defined only in an ``except`` block because the ``else`` block returned.
 

pylint/checkers/utils.py

@@ -23,8 +23,6 @@
 from astroid.context import InferenceContext
 from astroid.exceptions import AstroidError
 
-from pylint.constants import TYPING_TYPE_CHECKS_GUARDS
-
 if TYPE_CHECKING:
     from pylint.checkers import BaseChecker
 
@@ -1778,12 +1776,34 @@ def get_node_first_ancestor_of_type_and_its_child(
 
 
 def in_type_checking_block(node: nodes.NodeNG) -> bool:
-    """Check if a node is guarded by a TYPE_CHECKS guard."""
-    return any(
-        isinstance(ancestor, nodes.If)
-        and ancestor.test.as_string() in TYPING_TYPE_CHECKS_GUARDS
-        for ancestor in node.node_ancestors()
-    )
+    """Check if a node is guarded by a TYPE_CHECKING guard."""
+    for ancestor in node.node_ancestors():
+        if not isinstance(ancestor, nodes.If):
+            continue
+        if isinstance(ancestor.test, nodes.Name):
+            if ancestor.test.name != "TYPE_CHECKING":
+                continue
+            maybe_import_from = ancestor.test.lookup(ancestor.test.name)[1][0]
+            if (
+                isinstance(maybe_import_from, nodes.ImportFrom)
+                and maybe_import_from.modname == "typing"
+            ):
+                return True
+            inferred = safe_infer(ancestor.test)
+            if isinstance(inferred, nodes.Const) and inferred.value is False:
+                return True
+        elif isinstance(ancestor.test, nodes.Attribute):
+            if ancestor.test.attrname != "TYPE_CHECKING":
+                continue
+            inferred_module = safe_infer(ancestor.test.expr)
+            if (
+                isinstance(inferred_module, nodes.Module)
+                and inferred_module.name == "typing"
+                and inferred_module.pytype() == "builtins.module"
+            ):
+                return True
+
+    return False
 
 
 @lru_cache()

tests/functional/u/unused/unused_import.py

@@ -17,15 +17,18 @@ class SomeClass(object):
     SomeOtherName = SomeOtherName
 
 from never import __all__
-# pylint: disable=wrong-import-order,ungrouped-imports
+# pylint: disable=wrong-import-order,ungrouped-imports,reimported
 import typing
 from typing import TYPE_CHECKING
+import typing as t
 
 
 if typing.TYPE_CHECKING:
     import collections
 if TYPE_CHECKING:
     import itertools
+if t.TYPE_CHECKING:
+    import xml
 
 
 def get_ordered_dict() -> 'collections.OrderedDict':
@@ -65,3 +68,22 @@ def blop(self):
 if TYPE_CHECKING:
     if sys.version_info >= (3, 6, 2):
         from typing import NoReturn
+
+# Pathological cases
+from io import TYPE_CHECKING  # pylint: disable=no-name-in-module
+import trace as t
+import astroid as typing
+TYPE_CHECKING = "red herring"
+
+if TYPE_CHECKING:
+    import unittest  # [unused-import]
+if t.TYPE_CHECKING:  # pylint: disable=no-member
+    import uuid  # [unused-import]
+if typing.TYPE_CHECKING:  # pylint: disable=no-member
+    import warnings  # [unused-import]
+if typing.TYPE_CHECKING_WITH_MAGIC:  # pylint: disable=no-member
+    import compileall  # [unused-import]
+
+TYPE_CHECKING = False
+if TYPE_CHECKING:
+    import zoneinfo

tests/functional/u/unused/unused_import.txt

@@ -7,4 +7,8 @@ unused-import:9:0:9:51::Unused OrderedDict imported from collections:UNDEFINED
 unused-import:9:0:9:51::Unused deque imported from collections:UNDEFINED
 unused-import:10:0:10:22::Unused import re:UNDEFINED
 unused-import:13:0:13:40::Unused SomeOtherName imported from fake:UNDEFINED
-unused-import:45:0:45:9::Unused import os:UNDEFINED
+unused-import:48:0:48:9::Unused import os:UNDEFINED
+unused-import:79:4:79:19::Unused import unittest:UNDEFINED
+unused-import:81:4:81:15::Unused import uuid:UNDEFINED
+unused-import:83:4:83:19::Unused import warnings:UNDEFINED
+unused-import:85:4:85:21::Unused import compileall:UNDEFINED