Adds does_not_raise context manager

Addressing issues #4324 and #1830

Author: arel

AUTHORS

@@ -27,6 +27,7 @@ Anthony Shaw
 Anthony Sottile
 Anton Lodder
 Antony Lee
+Arel Cordero
 Armin Rigo
 Aron Coyle
 Aron Curzon

src/_pytest/python_api.py

@@ -4,6 +4,7 @@
 import pprint
 import sys
 import warnings
+from contextlib import contextmanager
 from decimal import Decimal
 from numbers import Number
 
@@ -726,3 +727,32 @@ def __exit__(self, *tp):
         if self.match_expr is not None and suppress_exception:
             self.excinfo.match(self.match_expr)
         return suppress_exception
+
+
+# builtin pytest.does_not_raise helper
+
+
+@contextmanager
+def does_not_raise():
+    r"""
+    This context manager is a complement to ``pytest.raises()`` that does
+    *not* catch any exceptions raised by the code block.
+
+
+    This is essentially a no-op but is useful when
+    conditionally parameterizing tests that may or may not
+    raise an error. For example::
+
+        @pytest.mark.parametrize('example_input,expectation', [
+            (3, does_not_raise()),
+            (2, does_not_raise()),
+            (1, does_not_raise()),
+            (0, raises(ZeroDivisionError)),
+        ])
+        def test_division(example_input, expectation):
+            '''Test how much I know division.'''
+            with expectation:
+                assert (6 / example_input) is not None
+    """
+
+    yield

src/pytest.py

@@ -32,6 +32,7 @@
 from _pytest.python import Module
 from _pytest.python import Package
 from _pytest.python_api import approx
+from _pytest.python_api import does_not_raise
 from _pytest.python_api import raises
 from _pytest.recwarn import deprecated_call
 from _pytest.recwarn import warns
@@ -50,6 +51,7 @@
     "cmdline",
     "Collector",
     "deprecated_call",
+    "does_not_raise",
     "exit",
     "fail",
     "File",

testing/python/raises.py

@@ -94,6 +94,46 @@ def test_raise_wrong_exception_passes_by():
         result = testdir.runpytest()
         result.stdout.fnmatch_lines(["*3 passed*"])
 
+    def test_does_not_raise(self, testdir):
+        testdir.makepyfile(
+            """
+            import pytest
+            import _pytest._code
+
+            @pytest.mark.parametrize('example_input,expectation', [
+                (3, pytest.does_not_raise()),
+                (2, pytest.does_not_raise()),
+                (1, pytest.does_not_raise()),
+                (0, pytest.raises(ZeroDivisionError)),
+            ])
+            def test_division(example_input, expectation):
+                '''Test how much I know division.'''
+                with expectation:
+                    assert (6 / example_input) is not None
+        """
+        )
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines(["*4 passed*"])
+
+    def test_does_not_raise_does_raise(self, testdir):
+        testdir.makepyfile(
+            """
+            import pytest
+            import _pytest._code
+
+            @pytest.mark.parametrize('example_input,expectation', [
+                (0, pytest.does_not_raise()),
+                (1, pytest.raises(ZeroDivisionError)),
+            ])
+            def test_division(example_input, expectation):
+                '''Test how much I know division.'''
+                with expectation:
+                    assert (6 / example_input) is not None
+        """
+        )
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines(["*2 failed*"])
+
     def test_noclass(self):
         with pytest.raises(TypeError):
             pytest.raises("wrong", lambda: None)