Add tests for SyntaxError with top-level return

IPython/core/tests/test_async_helpers.py

@@ -6,11 +6,12 Should only trigger on python 3.5+ or will have syntax errors.
 
 import sys
 import nose.tools as nt
-from textwrap import dedent
+from textwrap import dedent, indent
 from unittest import TestCase
 
 ip = get_ipython()
 iprc = lambda x: ip.run_cell(dedent(x))
+iprc_err = lambda x: iprc(x).raise_error()
 
 if sys.version_info > (3, 5):
     from IPython.core.async_helpers import _should_be_async
@@ -31,6 +32,183 if sys.version_info > (3, 5):
                 )
             )
 
+        def _get_top_level_cases(self):
+            # These are test cases that should be valid in a function
+            # but invalid outside of a function.
+            test_cases = []
+            test_cases.append(('basic', "{val}"))
+
+            # Note, in all conditional cases, I use True instead of
+            # False so that the peephole optimizer won't optimize away
+            # the return, so CPython will see this as a syntax error:
+            #
+            # while True:
+            #    break
+            #    return
+            #
+            # But not this:
+            #
+            # while False:
+            #    return
+            #
+            # See https://bugs.python.org/issue1875
+
+            test_cases.append(('if', dedent("""
+            if True:
+                {val}
+            """)))
+
+            test_cases.append(('while', dedent("""
+            while True:
+                {val}
+                break
+            """)))
+
+            test_cases.append(('try', dedent("""
+            try:
+                {val}
+            except:
+                pass
+            """)))
+
+            test_cases.append(('except', dedent("""
+            try:
+                pass
+            except:
+                {val}
+            """)))
+
+            test_cases.append(('finally', dedent("""
+            try:
+                pass
+            except:
+                pass
+            finally:
+                {val}
+            """)))
+
+            test_cases.append(('for', dedent("""
+            for _ in range(4):
+                {val}
+            """)))
+
+
+            test_cases.append(('nested', dedent("""
+            if True:
+                while True:
+                    {val}
+                    break
+            """)))
+
+            test_cases.append(('deep-nested', dedent("""
+            if True:
+                while True:
+                    break
+                    for x in range(3):
+                        if True:
+                            while True:
+                                for x in range(3):
+                                    {val}
+            """)))
+
+            return test_cases
+
+        def _get_ry_syntax_errors(self):
+            # This is a mix of tests that should be a syntax error if
+            # return or yield whether or not they are in a function
+
+            test_cases = []
+
+            test_cases.append(('class', dedent("""
+            class V:
+                {val}
+            """)))
+
+            test_cases.append(('nested-class', dedent("""
+            class V:
+                class C:
+                    {val}
+            """)))
+
+            return test_cases
+
+
+        def test_top_level_return_error(self):
+            tl_err_test_cases = self._get_top_level_cases()
+            tl_err_test_cases.extend(self._get_ry_syntax_errors())
+
+            vals = ('return', 'yield', 'yield from (_ for _ in range(3))')
+
+            for test_name, test_case in tl_err_test_cases:
+                # This example should work if 'pass' is used as the value
+                with self.subTest((test_name, 'pass')):
+                    iprc_err(test_case.format(val='pass'))
+
+                # It should fail with all the values
+                for val in vals:
+                    with self.subTest((test_name, val)):
+                        msg = "Syntax error not raised for %s, %s" % (test_name, val)
+                        with self.assertRaises(SyntaxError, msg=msg):
+                            iprc_err(test_case.format(val=val))
+
+        def test_in_func_no_error(self):
+            # Test that the implementation of top-level return/yield
+            # detection isn't *too* aggressive, and works inside a function
+            func_contexts = []
+
+            func_contexts.append(('func', dedent("""
+            def f():""")))
+
+            func_contexts.append(('method', dedent("""
+            class MyClass:
+                def __init__(self):
+            """)))
+
+            func_contexts.append(('async-func', dedent("""
+            async def f():""")))
+
+            func_contexts.append(('closure', dedent("""
+            def f():
+                def g():
+            """)))
+
+            def nest_case(context, case):
+                # Detect indentation
+                lines = context.strip().splitlines()
+                prefix_len = 0
+                for c in lines[-1]:
+                    if c != ' ':
+                        break
+                    prefix_len += 1
+
+                indented_case = indent(case, ' ' * (prefix_len + 4))
+                return context + '\n' + indented_case
+
+            # Gather and run the tests
+            vals = ('return', 'yield')
+
+            success_tests = self._get_top_level_cases()
+            failure_tests = self._get_ry_syntax_errors()
+
+            for context_name, context in func_contexts:
+                # These tests should now successfully run
+                for test_name, test_case in success_tests:
+                    nested_case = nest_case(context, test_case)
+
+                    for val in vals:
+                        with self.subTest((test_name, context_name, val)):
+                            iprc_err(nested_case.format(val=val))
+
+                # These tests should still raise a SyntaxError
+                for test_name, test_case in failure_tests:
+                    nested_case = nest_case(context, test_case)
+
+                    for val in vals:
+                        with self.subTest((test_name, context_name, val)):
+                            with self.assertRaises(SyntaxError):
+                                iprc_err(nested_case.format(val=val))
+
+
         def test_execute(self):
             iprc(
                 """