Consider ``range()`` objects for ``undefined-loop-variable`` leaking from iteration.

Close #2533

7 files changed, 45 insertions, 22 deletions

diff --git a/ChangeLog b/ChangeLog
index 962141f2f..51f34f002 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -7,6 +7,10 @@ What's New in Pylint 2.2?
 
 Release date: TBA
 
+   * Consider ``range()`` objects for ``undefined-loop-variable`` leaking from iteration.
+
+     Close #2533
+
    * ``deprecated-method`` can use the attribute name for identifying a deprecated method
 
      Previously we were using the fully qualified name, which we still do, but the fully
diff --git a/pylint/checkers/format.py b/pylint/checkers/format.py
index b5f35cd00..38b6649c5 100644
--- a/pylint/checkers/format.py
+++ b/pylint/checkers/format.py
@@ -1333,3 +1333,9 @@ class FormatChecker(BaseTokenChecker):
 def register(linter):
     """required method to auto register this checker """
     linter.register_checker(FormatChecker(linter))
+
+
+def funkier():
+    for val in range(3):
+        pass
+    print(val)
diff --git a/pylint/checkers/variables.py b/pylint/checkers/variables.py
index 78d78a913..784568ddd 100644
--- a/pylint/checkers/variables.py
+++ b/pylint/checkers/variables.py
@@ -64,6 +64,7 @@ PY3K = sys.version_info >= (3, 0)
 # for `abc`
 METACLASS_NAME_TRANSFORMS = {"_py_abc": "abc"}
 TYPING_TYPE_CHECKS_GUARDS = frozenset({"typing.TYPE_CHECKING", "TYPE_CHECKING"})
+BUILTIN_RANGE = "builtins.range"
 
 
 def _is_from_future_import(stmt, name):
@@ -1145,7 +1146,7 @@ class VariablesChecker(BaseChecker):
         ):
             return
 
-        # For functions we can do more by inferring the length of the iterred object
+        # For functions we can do more by inferring the length of the itered object
         if not isinstance(assign, astroid.For):
             self.add_message("undefined-loop-variable", args=name, node=node)
             return
@@ -1155,6 +1156,14 @@ class VariablesChecker(BaseChecker):
         except astroid.InferenceError:
             self.add_message("undefined-loop-variable", args=name, node=node)
         else:
+            if (
+                isinstance(inferred, astroid.Instance)
+                and inferred.qname() == BUILTIN_RANGE
+            ):
+                # Consider range() objects safe, even if they might not yield any results.
+                return
+
+            # Consider sequences.
             sequences = (
                 astroid.List,
                 astroid.Tuple,
diff --git a/pylint/test/functional/cellvar_escaping_loop.py b/pylint/test/functional/cellvar_escaping_loop.py
index 29107b575..23c6b4b2e 100644
--- a/pylint/test/functional/cellvar_escaping_loop.py
+++ b/pylint/test/functional/cellvar_escaping_loop.py
@@ -35,16 +35,6 @@ def good_case5():
 
 
 def good_case6():
-    """Accept use of the variable after the loop.
-
-    There's already a warning about possibly undefined loop variables, and
-    the value will not change any more."""
-    for i in range(10):
-        print(i)
-    return lambda: i  # [undefined-loop-variable]
-
-
-def good_case7():
     """Accept use of the variable inside return."""
     for i in range(10):
         if i == 8:
@@ -52,13 +42,13 @@ def good_case7():
     return lambda: -1
 
 
-def good_case8():
+def good_case7():
     """Lambda defined and called in loop."""
     for i in range(10):
         print((lambda x: i + x)(1))
 
 
-def good_case9():
+def good_case8():
     """Another eager binding of the cell variable."""
     funs = []
     for i in range(10):
diff --git a/pylint/test/functional/cellvar_escaping_loop.txt b/pylint/test/functional/cellvar_escaping_loop.txt
index bdd7fe9c6..2ac0b58ef 100644
--- a/pylint/test/functional/cellvar_escaping_loop.txt
+++ b/pylint/test/functional/cellvar_escaping_loop.txt
@@ -1,7 +1,6 @@
-undefined-loop-variable:44:good_case6.<lambda>:Using possibly undefined loop variable 'i'
-cell-var-from-loop:77:bad_case.<lambda>:Cell variable i defined in loop
-cell-var-from-loop:82:bad_case2.<lambda>:Cell variable i defined in loop
-cell-var-from-loop:90:bad_case3.<lambda>:Cell variable j defined in loop
-cell-var-from-loop:100:bad_case4.nested:Cell variable i defined in loop
-cell-var-from-loop:121:bad_case5.<lambda>:Cell variable i defined in loop
-cell-var-from-loop:129:bad_case6.<lambda>:Cell variable i defined in loop
+cell-var-from-loop:67:bad_case.<lambda>:Cell variable i defined in loop
+cell-var-from-loop:72:bad_case2.<lambda>:Cell variable i defined in loop
+cell-var-from-loop:80:bad_case3.<lambda>:Cell variable j defined in loop
+cell-var-from-loop:90:bad_case4.nested:Cell variable i defined in loop
+cell-var-from-loop:111:bad_case5.<lambda>:Cell variable i defined in loop
+cell-var-from-loop:119:bad_case6.<lambda>:Cell variable i defined in loop
diff --git a/pylint/test/functional/undefined_loop_variable.py b/pylint/test/functional/undefined_loop_variable.py
index 3840003b5..a984898f8 100644
--- a/pylint/test/functional/undefined_loop_variable.py
+++ b/pylint/test/functional/undefined_loop_variable.py
@@ -1,4 +1,4 @@
-# pylint: disable=missing-docstring
+# pylint: disable=missing-docstring,redefined-builtin
 
 def do_stuff(some_random_list):
     for var in some_random_list:
@@ -59,3 +59,17 @@ def do_stuff_with_a_tuple():
     for var in (1, 2, 3):
         pass
     return var
+
+
+def do_stuff_with_a_range():
+    for var in range(1, 2):
+        pass
+    return var
+
+
+def do_stuff_with_redefined_range():
+    def range(key):
+        yield from [1, key]
+    for var in range(3):
+        pass
+    return var # [undefined-loop-variable]
diff --git a/pylint/test/functional/undefined_loop_variable.txt b/pylint/test/functional/undefined_loop_variable.txt
index 33022d454..64cf6388b 100644
--- a/pylint/test/functional/undefined_loop_variable.txt
+++ b/pylint/test/functional/undefined_loop_variable.txt
@@ -1,2 +1,3 @@
 undefined-loop-variable:6:do_stuff:Using possibly undefined loop variable 'var'
-undefined-loop-variable:25::Using possibly undefined loop variable 'var1'
\ No newline at end of file
+undefined-loop-variable:25::Using possibly undefined loop variable 'var1'
+undefined-loop-variable:75:do_stuff_with_redefined_range:Using possibly undefined loop variable 'var'