Add unguarded-next-without-default

doc/data/messages/u/unguarded-next-without-default/bad.py

@@ -0,0 +1 @@
+next(i for i in (1, 2) if isinstance(i, str))  # [unspecified-default-for-next]

doc/data/messages/u/unguarded-next-without-default/good.py

@@ -0,0 +1 @@
+next((i for i in (1, 2) if isinstance(i, str)), None)

doc/data/messages/u/unguarded-next-without-default/related.rst

@@ -0,0 +1 @@
+- `PEP 479 <https://peps.python.org/pep-0479/>`_

doc/whatsnew/2/2.15/index.rst

@@ -15,6 +15,10 @@ Summary -- Release highlights
 New checkers
 ============
 
+* Added ``unguarded-next-without-default`` checker which warns about calls to ``next()``
+  without a default value.
+
+  Closes #4725
 
 Removed checkers
 ================

pylint/checkers/stdlib.py

@@ -434,6 +434,13 @@ class StdlibChecker(DeprecatedMixin, BaseChecker):
                 ]
             },
         ),
+        "W1519": (
+            "Using next without explicitly specifying a default value or catching the StopIteration",
+            "unguarded-next-without-default",
+            "Without a default value calls to next() can raise a StopIteration "
+            "exception. This exception should be caught or a default value can "
+            "be provided.",
+        ),
     }
 
     def __init__(self, linter: PyLinter) -> None:
@@ -498,6 +505,7 @@ def _check_shallow_copy_environ(self, node: nodes.Call) -> None:
         "deprecated-class",
         "unspecified-encoding",
         "forgotten-debug-statement",
+        "unguarded-next-without-default",
     )
     def visit_call(self, node: nodes.Call) -> None:
         """Visit a Call node."""
@@ -522,6 +530,7 @@ def visit_call(self, node: nodes.Call) -> None:
                     self._check_for_preexec_fn_in_popen(node)
             elif isinstance(inferred, nodes.FunctionDef):
                 name = inferred.qname()
+                self._check_next_call(node, name)
                 if name == COPY_COPY:
                     self._check_shallow_copy_environ(node)
                 elif name in ENV_GETTERS:
@@ -720,6 +729,24 @@ def _check_env_function(self, node, infer):
                 allow_none=True,
             )
 
+    def _check_next_call(self, node: nodes.Call, name: str) -> None:
+        if name != "builtins.next":
+            return
+        # We don't care about this call if there are zero arguments
+        if len(node.args) != 1:
+            return
+        if utils.get_exception_handlers(node, StopIteration):
+            return
+        # Raising is fine within __next__
+        func_def = utils.get_node_first_ancestor_of_type(node, nodes.FunctionDef)
+        if func_def and func_def.name == "__next__":
+            return
+        self.add_message(
+            "unguarded-next-without-default",
+            node=node,
+            confidence=interfaces.INFERENCE,
+        )
+
     def _check_invalid_envvar_value(self, node, infer, message, call_arg, allow_none):
         if call_arg in (astroid.Uninferable, None):
             return

pylintrc

@@ -104,6 +104,8 @@ disable=
     format,
     # We anticipate #3512 where it will become optional
     fixme,
+    # TODO: 2.15: Fix and enable.
+    unguarded-next-without-default
 
 
 [REPORTS]

tests/functional/c/cellvar_escaping_loop.py

@@ -1,4 +1,5 @@
-# pylint: disable=unnecessary-comprehension,missing-docstring,too-few-public-methods,unnecessary-direct-lambda-call
+# pylint: disable=unnecessary-comprehension, missing-docstring, too-few-public-methods
+# pylint: disable=unnecessary-direct-lambda-call, unguarded-next-without-default
 """Tests for loopvar-in-closure."""
 from __future__ import print_function
 
@@ -55,9 +56,11 @@ def good_case8():
     """Another eager binding of the cell variable."""
     funs = []
     for i in range(10):
+
         def func(bound_i=i):
             """Ignore."""
             return bound_i
+
         funs.append(func)
     return funs
 
@@ -75,8 +78,10 @@ def good_case10():
     """Ignore when a loop variable is shadowed by an inner function"""
     lst = []
     for i in range(10):  # pylint: disable=unused-variable
+
         def func():
             i = 100
+
             def func2(arg=i):
                 return arg
 
@@ -87,8 +92,7 @@ def func2(arg=i):
 
 
 def good_case_issue3107():
-    """Eager binding of cell variable when used in a non-trivial default argument expression.
-    """
+    """Eager binding of cell variable when used in a non-trivial default argument expression."""
     for i in [[2], [3]]:
         next(filter(lambda j, ix=i[0]: j == ix, [1, 3]))
 
@@ -99,16 +103,20 @@ def good_case_issue_5012():
     """
     funs = []
     for i in range(5):
+
         def func(*, _i=i):
             print(_i)
+
         funs.append(func)
 
         def func2(_i=i):
             print(_i)
+
         funs.append(func2)
 
     return funs
 
+
 def bad_case():
     """Closing over a loop variable."""
     lst = []
@@ -135,9 +143,11 @@ def bad_case4():
     """Closing over variable defined in loop."""
     lst = []
     for i in range(10):
+
         def nested():
             """Nested function."""
             return i**2  # [cell-var-from-loop]
+
         lst.append(nested)
     return lst
 
@@ -173,9 +183,7 @@ def bad_case6():
 def bad_case7():
     """Multiple variables unpacked in comprehension."""
     return [
-        lambda: (
-            x  # [cell-var-from-loop]
-            + y)  # [cell-var-from-loop]
+        lambda: (x + y)  # [cell-var-from-loop]  # [cell-var-from-loop]
         for x, y in ((1, 2), (3, 4), (5, 6))
     ]
 
@@ -202,6 +210,7 @@ def bad_case10():
     """Detect when a loop variable is the default argument for a nested function"""
     lst = []
     for i in range(10):
+
         def func():
             def func2(arg=i):  # [cell-var-from-loop]
                 return arg
@@ -214,15 +223,9 @@ def func2(arg=i):  # [cell-var-from-loop]
 
 def bad_case_issue2846():
     """Closing over variable that is used within a comprehension in the function body."""
-    lst_a = [
-        (lambda: n)  # [cell-var-from-loop]
-        for n in range(3)
-    ]
+    lst_a = [(lambda: n) for n in range(3)]  # [cell-var-from-loop]
 
-    lst_b = [
-        (lambda: [n for _ in range(3)])  # [cell-var-from-loop]
-        for n in range(3)
-    ]
+    lst_b = [(lambda: [n for _ in range(3)]) for n in range(3)]  # [cell-var-from-loop]
 
     return lst_a, lst_b
 

tests/functional/c/cellvar_escaping_loop.txt

@@ -1,13 +1,13 @@
-cell-var-from-loop:117:27:117:28:bad_case.<lambda>:Cell variable i defined in loop:UNDEFINED
-cell-var-from-loop:122:20:122:21:bad_case2.<lambda>:Cell variable i defined in loop:UNDEFINED
-cell-var-from-loop:130:27:130:28:bad_case3.<lambda>:Cell variable j defined in loop:UNDEFINED
-cell-var-from-loop:140:19:140:20:bad_case4.nested:Cell variable i defined in loop:UNDEFINED
-cell-var-from-loop:161:20:161:21:bad_case5.<lambda>:Cell variable i defined in loop:UNDEFINED
-cell-var-from-loop:169:27:169:28:bad_case6.<lambda>:Cell variable i defined in loop:UNDEFINED
-cell-var-from-loop:177:12:177:13:bad_case7.<lambda>:Cell variable x defined in loop:UNDEFINED
-cell-var-from-loop:178:14:178:15:bad_case7.<lambda>:Cell variable y defined in loop:UNDEFINED
-cell-var-from-loop:187:27:187:28:bad_case8.<lambda>:Cell variable j defined in loop:UNDEFINED
-cell-var-from-loop:197:27:197:28:bad_case9.<lambda>:Cell variable i defined in loop:UNDEFINED
-cell-var-from-loop:206:26:206:27:bad_case10.func.func2:Cell variable i defined in loop:UNDEFINED
-cell-var-from-loop:218:17:218:18:bad_case_issue2846.<lambda>:Cell variable n defined in loop:UNDEFINED
-cell-var-from-loop:223:18:223:19:bad_case_issue2846.<lambda>:Cell variable n defined in loop:UNDEFINED
+cell-var-from-loop:118:27:118:28:bad_case.<lambda>:Cell variable i defined in loop:UNDEFINED
+cell-var-from-loop:123:20:123:21:bad_case2.<lambda>:Cell variable i defined in loop:UNDEFINED
+cell-var-from-loop:131:27:131:28:bad_case3.<lambda>:Cell variable j defined in loop:UNDEFINED
+cell-var-from-loop:141:19:141:20:bad_case4.nested:Cell variable i defined in loop:UNDEFINED
+cell-var-from-loop:162:20:162:21:bad_case5.<lambda>:Cell variable i defined in loop:UNDEFINED
+cell-var-from-loop:170:27:170:28:bad_case6.<lambda>:Cell variable i defined in loop:UNDEFINED
+cell-var-from-loop:178:12:178:13:bad_case7.<lambda>:Cell variable x defined in loop:UNDEFINED
+cell-var-from-loop:179:14:179:15:bad_case7.<lambda>:Cell variable y defined in loop:UNDEFINED
+cell-var-from-loop:188:27:188:28:bad_case8.<lambda>:Cell variable j defined in loop:UNDEFINED
+cell-var-from-loop:198:27:198:28:bad_case9.<lambda>:Cell variable i defined in loop:UNDEFINED
+cell-var-from-loop:207:26:207:27:bad_case10.func.func2:Cell variable i defined in loop:UNDEFINED
+cell-var-from-loop:219:17:219:18:bad_case_issue2846.<lambda>:Cell variable n defined in loop:UNDEFINED
+cell-var-from-loop:224:18:224:19:bad_case_issue2846.<lambda>:Cell variable n defined in loop:UNDEFINED

tests/functional/s/stop_iteration_inside_generator.py

@@ -1,20 +1,24 @@
 """
 Test that no StopIteration is raised inside a generator
 """
-# pylint: disable=missing-docstring,invalid-name,import-error, try-except-raise, wrong-import-position,not-callable,raise-missing-from
+# pylint: disable=missing-docstring, invalid-name, import-error, try-except-raise, wrong-import-position
+# pylint: disable=not-callable, raise-missing-from, unguarded-next-without-default
 import asyncio
 
+
 class RebornStopIteration(StopIteration):
     """
     A class inheriting from StopIteration exception
     """
 
+
 # This one is ok
 def gen_ok():
     yield 1
     yield 2
     yield 3
 
+
 # pylint should warn about this one
 # because of a direct raising of StopIteration inside generator
 def gen_stopiter():
@@ -23,6 +27,7 @@ def gen_stopiter():
     yield 3
     raise StopIteration  # [stop-iteration-return]
 
+
 # pylint should warn about this one
 # because of a direct raising of an exception inheriting from StopIteration inside generator
 def gen_stopiterchild():
@@ -31,13 +36,15 @@ def gen_stopiterchild():
     yield 3
     raise RebornStopIteration  # [stop-iteration-return]
 
+
 # pylint should warn here
 # because of the possibility that next raises a StopIteration exception
 def gen_next_raises_stopiter():
     g = gen_ok()
     while True:
         yield next(g)  # [stop-iteration-return]
 
+
 # This one is the same as gen_next_raises_stopiter
 # but is ok because the next function is inside
 # a try/except block handling StopIteration
@@ -49,6 +56,7 @@ def gen_next_inside_try_except():
         except StopIteration:
             return
 
+
 # This one is the same as gen_next_inside_try_except
 # but is not ok because the next function is inside
 # a try/except block that don't handle StopIteration
@@ -60,6 +68,7 @@ def gen_next_inside_wrong_try_except():
         except ValueError:
             return
 
+
 # This one is the same as gen_next_inside_try_except
 # but is not ok because the next function is inside
 # a try/except block that handle StopIteration but reraise it
@@ -71,11 +80,13 @@ def gen_next_inside_wrong_try_except2():
         except StopIteration:
             raise StopIteration  # [stop-iteration-return]
 
+
 # Those two last are ok
 def gen_in_for():
     for el in gen_ok():
         yield el
 
+
 def gen_yield_from():
     yield from gen_ok()
 
@@ -84,7 +95,7 @@ def gen_dont_crash_on_no_exception():
     g = gen_ok()
     while True:
         try:
-            yield next(g) # [stop-iteration-return]
+            yield next(g)  # [stop-iteration-return]
         except ValueError:
             raise
 
@@ -97,11 +108,12 @@ def gen_dont_crash_on_uninferable():
 
 # https://github.com/PyCQA/pylint/issues/1830
 def gen_next_with_sentinel():
-    yield next([], 42) # No bad return
+    yield next([], 42)  # No bad return
 
 
 from itertools import count
 
+
 # https://github.com/PyCQA/pylint/issues/2158
 def generator_using_next():
     counter = count()
@@ -113,6 +125,7 @@ def generator_using_next():
 class SomeClassWithNext:
     def next(self):
         return iter([1, 2, 3])
+
     def some_gen(self):
         for value in self.next():
             yield value
@@ -122,8 +135,8 @@ def some_gen(self):
 
 
 def something_invalid():
-    raise Exception('cannot iterate this')
+    raise Exception("cannot iterate this")
 
 
 def invalid_object_passed_to_next():
-    yield next(something_invalid()) # [stop-iteration-return]
+    yield next(something_invalid())  # [stop-iteration-return]

tests/functional/s/stop_iteration_inside_generator.txt

@@ -1,7 +1,7 @@
-stop-iteration-return:24:4:24:23:gen_stopiter:Do not raise StopIteration in generator, use return statement instead:UNDEFINED
-stop-iteration-return:32:4:32:29:gen_stopiterchild:Do not raise StopIteration in generator, use return statement instead:UNDEFINED
-stop-iteration-return:39:14:39:21:gen_next_raises_stopiter:Do not raise StopIteration in generator, use return statement instead:UNDEFINED
-stop-iteration-return:59:18:59:25:gen_next_inside_wrong_try_except:Do not raise StopIteration in generator, use return statement instead:UNDEFINED
-stop-iteration-return:72:12:72:31:gen_next_inside_wrong_try_except2:Do not raise StopIteration in generator, use return statement instead:UNDEFINED
-stop-iteration-return:87:18:87:25:gen_dont_crash_on_no_exception:Do not raise StopIteration in generator, use return statement instead:UNDEFINED
-stop-iteration-return:129:10:129:35:invalid_object_passed_to_next:Do not raise StopIteration in generator, use return statement instead:UNDEFINED
+stop-iteration-return:25:4:25:23:gen_stopiter:Do not raise StopIteration in generator, use return statement instead:UNDEFINED
+stop-iteration-return:33:4:33:29:gen_stopiterchild:Do not raise StopIteration in generator, use return statement instead:UNDEFINED
+stop-iteration-return:40:14:40:21:gen_next_raises_stopiter:Do not raise StopIteration in generator, use return statement instead:UNDEFINED
+stop-iteration-return:60:18:60:25:gen_next_inside_wrong_try_except:Do not raise StopIteration in generator, use return statement instead:UNDEFINED
+stop-iteration-return:73:12:73:31:gen_next_inside_wrong_try_except2:Do not raise StopIteration in generator, use return statement instead:UNDEFINED
+stop-iteration-return:88:18:88:25:gen_dont_crash_on_no_exception:Do not raise StopIteration in generator, use return statement instead:UNDEFINED
+stop-iteration-return:130:10:130:35:invalid_object_passed_to_next:Do not raise StopIteration in generator, use return statement instead:UNDEFINED