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))  # [unguarded-next-without-default]

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
 

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

@@ -1,7 +1,8 @@
 """
 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):

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

tests/functional/u/undefined/undefined_variable_py38.py

@@ -1,5 +1,5 @@
 """Tests for undefined variable with assignment expressions"""
-# pylint: disable=using-constant-test, expression-not-assigned
+# pylint: disable=using-constant-test, expression-not-assigned, unguarded-next-without-default
 
 # Tests for annotation of variables and potentially undefinition
 

tests/functional/u/unguarded_next_without_default.py

@@ -0,0 +1,46 @@
+"""Warnings for using next() without specifying a default value."""
+# pylint: disable=missing-class-docstring, too-few-public-methods, missing-function-docstring
+# pylint: disable=inconsistent-return-statements
+
+next((i for i in (1, 2)), None)
+next(i for i in (1, 2))  # [unguarded-next-without-default]
+var = next(i for i in (1, 2))  # [unguarded-next-without-default]
+
+try:
+    next(i for i in (1, 2))
+except StopIteration:
+    pass
+
+try:
+    next(i for i in (1, 2))  # [unguarded-next-without-default]
+except ValueError:
+    pass
+
+try:
+    next(i for i in (1, 2))
+except (ValueError, StopIteration):
+    pass
+
+try:
+    next(i for i in (1, 2))
+except ValueError:
+    pass
+except StopIteration:
+    pass
+
+redefined_next = next
+redefined_next(i for i in (1, 2))  # [unguarded-next-without-default]
+
+
+class MyClass:
+    def __next__(self):
+        return next(i for i in (1, 2))
+
+
+# Example based on astroid code
+def func(keywords, context):
+    for value in keywords:
+        try:
+            return next(value.infer(context=context))
+        except StopIteration:
+            continue

tests/functional/u/unguarded_next_without_default.txt

@@ -0,0 +1,4 @@
+unguarded-next-without-default:6:0:6:23::Using next without explicitly specifying a default value or catching the StopIteration:INFERENCE
+unguarded-next-without-default:7:6:7:29::Using next without explicitly specifying a default value or catching the StopIteration:INFERENCE
+unguarded-next-without-default:15:4:15:27::Using next without explicitly specifying a default value or catching the StopIteration:INFERENCE
+unguarded-next-without-default:32:0:32:33::Using next without explicitly specifying a default value or catching the StopIteration:INFERENCE