Deprecate the 'message' parameter of pytest.raises

Fix #3974

Author: nicoddemus

changelog/3974.deprecation.rst

@@ -0,0 +1,8 @@
+Passing the ``message`` parameter of ``pytest.raises`` now issues a ``DeprecationWarning``.
+
+It is a common mistake to think this parameter will match the exception message, while in fact
+it only serves to provide a custom message in case the ``pytest.raises`` check fails. To avoid this
+mistake and because it is believed to be little used, pytest is deprecating it without providing
+an alternative for the moment.
+
+If you have concerns about this, please comment on `issue #3974 <https://github.com/pytest-dev/pytest/issues/3974>`__.

doc/en/deprecations.rst

@@ -14,6 +14,19 @@ Below is a complete list of all pytest features which are considered deprecated.
 :class:`_pytest.warning_types.PytestWarning` or subclasses, which can be filtered using
 :ref:`standard warning filters <warnings>`.
 
+``"message"`` parameter of ``pytest.raises``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. deprecated:: 4.1
+
+It is a common mistake to think this parameter will match the exception message, while in fact
+it only serves to provide a custom message in case the ``pytest.raises`` check fails. To avoid this
+mistake and because it is believed to be little used, pytest is deprecating it without providing
+an alternative for the moment.
+
+If you have concerns about this, please comment on `issue #3974 <https://github.com/pytest-dev/pytest/issues/3974>`__.
+
+
 ``pytest.config`` global
 ~~~~~~~~~~~~~~~~~~~~~~~~
 

src/_pytest/deprecated.py

@@ -51,6 +51,13 @@
     "getfuncargvalue is deprecated, use getfixturevalue"
 )
 
+RAISES_MESSAGE_PARAMETER = PytestDeprecationWarning(
+    "The 'message' parameter is deprecated.\n"
+    "(did you mean to use `match='some regex'` to check the exception message?)\n"
+    "Please comment on https://github.com/pytest-dev/pytest/issues/3974 "
+    "if you have concerns about removal of this parameter."
+)
+
 RESULT_LOG = PytestDeprecationWarning(
     "--result-log is deprecated and scheduled for removal in pytest 5.0.\n"
     "See https://docs.pytest.org/en/latest/deprecations.html#result-log-result-log for more information."

src/_pytest/python_api.py

@@ -13,11 +13,11 @@
 from six.moves import zip
 
 import _pytest._code
+from _pytest import deprecated
 from _pytest.compat import isclass
 from _pytest.compat import Mapping
 from _pytest.compat import Sequence
 from _pytest.compat import STRING_TYPES
-from _pytest.deprecated import RAISES_EXEC
 from _pytest.outcomes import fail
 
 BASE_TYPE = (type, STRING_TYPES)
@@ -551,29 +551,47 @@ def _is_numpy_array(obj):
 def raises(expected_exception, *args, **kwargs):
     r"""
     Assert that a code block/function call raises ``expected_exception``
-    and raise a failure exception otherwise.
+    or raise a failure exception otherwise.
 
-    :arg message: if specified, provides a custom failure message if the
-        exception is not raised
-    :arg match: if specified, asserts that the exception matches a text or regex
+    :kwparam match: if specified, asserts that the exception matches a text or regex
 
-    This helper produces a ``ExceptionInfo()`` object (see below).
+    :kwparam message: **(deprecated since 4.1)** if specified, provides a custom failure message
+        if the exception is not raised
 
-    You may use this function as a context manager::
+    .. currentmodule:: _pytest._code
+
+    Use ``pytest.raises`` as a context manager, which will capture the exception of the given
+    type::
 
         >>> with raises(ZeroDivisionError):
         ...    1/0
 
-    .. versionchanged:: 2.10
+    If the code block does not raise the expected exception (``ZeroDivisionError`` in the example
+    above), or no exception at all, the check will fail instead.
+
+    You can also use the keyword argument ``match`` to assert that the
+    exception matches a text or regex::
+
+        >>> with raises(ValueError, match='must be 0 or None'):
+        ...     raise ValueError("value must be 0 or None")
+
+        >>> with raises(ValueError, match=r'must be \d+$'):
+        ...     raise ValueError("value must be 42")
 
-    In the context manager form you may use the keyword argument
-    ``message`` to specify a custom failure message::
+    The context manager produces an :class:`ExceptionInfo` object which can be used to inspect the
+    details of the captured exception::
 
-        >>> with raises(ZeroDivisionError, message="Expecting ZeroDivisionError"):
-        ...    pass
-        Traceback (most recent call last):
-          ...
-        Failed: Expecting ZeroDivisionError
+        >>> with raises(ValueError) as exc_info:
+        ...     raise ValueError("value must be 42")
+        >>> assert exc_info.type is ValueError
+        >>> assert exc_info.value.args[0] == "value must be 42"
+
+    .. deprecated:: 4.1
+
+        In the context manager form you may use the keyword argument
+        ``message`` to specify a custom failure message that will be displayed
+        in case the ``pytest.raises`` check fails. This has been deprecated as it
+        is considered error prone as users often mean to use ``match`` instead.
 
     .. note::
 
@@ -587,7 +605,7 @@ def raises(expected_exception, *args, **kwargs):
            >>> with raises(ValueError) as exc_info:
            ...     if value > 10:
            ...         raise ValueError("value must be <= 10")
-           ...     assert exc_info.type == ValueError  # this will not execute
+           ...     assert exc_info.type is ValueError  # this will not execute
 
        Instead, the following approach must be taken (note the difference in
        scope)::
@@ -596,23 +614,10 @@ def raises(expected_exception, *args, **kwargs):
            ...     if value > 10:
            ...         raise ValueError("value must be <= 10")
            ...
-           >>> assert exc_info.type == ValueError
-
-
-    Since version ``3.1`` you can use the keyword argument ``match`` to assert that the
-    exception matches a text or regex::
-
-        >>> with raises(ValueError, match='must be 0 or None'):
-        ...     raise ValueError("value must be 0 or None")
-
-        >>> with raises(ValueError, match=r'must be \d+$'):
-        ...     raise ValueError("value must be 42")
+           >>> assert exc_info.type is ValueError
 
     **Legacy form**
 
-    The form below is fully supported but discouraged for new code because the
-    context manager form is regarded as more readable and less error-prone.
-
     It is possible to specify a callable by passing a to-be-called lambda::
 
         >>> raises(ZeroDivisionError, lambda: 1/0)
@@ -627,9 +632,8 @@ def raises(expected_exception, *args, **kwargs):
         >>> raises(ZeroDivisionError, f, x=0)
         <ExceptionInfo ...>
 
-    .. currentmodule:: _pytest._code
-
-    Consult the API of ``excinfo`` objects: :class:`ExceptionInfo`.
+    The form above is fully supported but discouraged for new code because the
+    context manager form is regarded as more readable and less error-prone.
 
     .. note::
         Similar to caught exception objects in Python, explicitly clearing
@@ -660,6 +664,7 @@ def raises(expected_exception, *args, **kwargs):
     if not args:
         if "message" in kwargs:
             message = kwargs.pop("message")
+            warnings.warn(deprecated.RAISES_MESSAGE_PARAMETER, stacklevel=2)
         if "match" in kwargs:
             match_expr = kwargs.pop("match")
         if kwargs:
@@ -668,7 +673,7 @@ def raises(expected_exception, *args, **kwargs):
             raise TypeError(msg)
         return RaisesContext(expected_exception, message, match_expr)
     elif isinstance(args[0], str):
-        warnings.warn(RAISES_EXEC, stacklevel=2)
+        warnings.warn(deprecated.RAISES_EXEC, stacklevel=2)
         code, = args
         assert isinstance(code, str)
         frame = sys._getframe(1)

testing/deprecated_test.py

@@ -136,6 +136,12 @@ def test_func(pytestconfig):
     )
 
 
+def test_raises_message_argument_deprecated():
+    with pytest.warns(pytest.PytestDeprecationWarning):
+        with pytest.raises(RuntimeError, message="foobar"):
+            raise RuntimeError
+
+
 def test_pytest_plugins_in_non_top_level_conftest_deprecated(testdir):
     from _pytest.deprecated import PYTEST_PLUGINS_FROM_NON_TOP_LEVEL_CONFTEST
 

testing/python/raises.py

@@ -121,8 +121,9 @@ def test_no_raise_message(self):
     def test_custom_raise_message(self):
         message = "TEST_MESSAGE"
         try:
-            with pytest.raises(ValueError, message=message):
-                pass
+            with pytest.warns(PytestDeprecationWarning):
+                with pytest.raises(ValueError, message=message):
+                    pass
         except pytest.raises.Exception as e:
             assert e.msg == message
         else:

testing/test_pytester.py

@@ -280,7 +280,7 @@ def test_assert_outcomes_after_pytest_error(testdir):
     testdir.makepyfile("def test_foo(): assert True")
 
     result = testdir.runpytest("--unexpected-argument")
-    with pytest.raises(ValueError, message="Pytest terminal report not found"):
+    with pytest.raises(ValueError, match="Pytest terminal report not found"):
         result.assert_outcomes(passed=0)