allow error message matching in pytest.raises

Author: Kriechi

CHANGELOG.rst

@@ -13,6 +13,9 @@ New Features
 * ``pytest.warns`` now checks for subclass relationship rather than
   class equality. Thanks `@lesteve`_ for the PR (`#2166`_)
 
+* ``pytest.raises`` now asserts that the error message matches a text or regex
+  with the `match` keyword argument. Thanks `@Kriechi`_ for the PR.
+
 
 Changes
 -------
@@ -56,6 +59,7 @@ Changes
 .. _@fogo: https://github.com/fogo
 .. _@mandeep: https://github.com/mandeep
 .. _@unsignedint: https://github.com/unsignedint
+.. _@Kriechi: https://github.com/Kriechi
 
 .. _#1512: https://github.com/pytest-dev/pytest/issues/1512
 .. _#1874: https://github.com/pytest-dev/pytest/pull/1874

_pytest/python.py

@@ -1,5 +1,6 @@
 """ Python test discovery, setup and run of test functions. """
 
+import re
 import fnmatch
 import inspect
 import sys
@@ -1134,7 +1135,7 @@ def raises(expected_exception, *args, **kwargs):
            >>> with raises(ValueError) as exc_info:
            ...     if value > 10:
            ...         raise ValueError("value must be <= 10")
-           ...     assert str(exc_info.value) == "value must be <= 10"  # this will not execute
+           ...     assert exc_info.type == ValueError  # this will not execute
 
        Instead, the following approach must be taken (note the difference in
        scope)::
@@ -1143,7 +1144,16 @@ def raises(expected_exception, *args, **kwargs):
            ...     if value > 10:
            ...         raise ValueError("value must be <= 10")
            ...
-           >>> assert str(exc_info.value) == "value must be <= 10"
+           >>> assert exc_info.type == ValueError
+
+    Or 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")
 
 
     Or you can specify a callable by passing a to-be-called lambda::
@@ -1194,11 +1204,15 @@ def raises(expected_exception, *args, **kwargs):
         raise TypeError(msg % type(expected_exception))
 
     message = "DID NOT RAISE {0}".format(expected_exception)
+    match_expr = None
 
     if not args:
         if "message" in kwargs:
             message = kwargs.pop("message")
-        return RaisesContext(expected_exception, message)
+        if "match" in kwargs:
+            match_expr = kwargs.pop("match")
+            message += " matching '{0}'".format(match_expr)
+        return RaisesContext(expected_exception, message, match_expr)
     elif isinstance(args[0], str):
         code, = args
         assert isinstance(code, str)
@@ -1222,9 +1236,10 @@ def raises(expected_exception, *args, **kwargs):
     pytest.fail(message)
 
 class RaisesContext(object):
-    def __init__(self, expected_exception, message):
+    def __init__(self, expected_exception, message, match_expr):
         self.expected_exception = expected_exception
         self.message = message
+        self.match_expr = match_expr
         self.excinfo = None
 
     def __enter__(self):
@@ -1243,6 +1258,8 @@ def __exit__(self, *tp):
                 exc_type, value, traceback = tp
                 tp = exc_type, exc_type(value), traceback
         self.excinfo.__init__(tp)
+        if self.match_expr and not re.search(self.match_expr, str(self.excinfo)):
+            pytest.fail(self.message)
         suppress_exception = issubclass(self.excinfo.type, self.expected_exception)
         if sys.version_info[0] == 2 and suppress_exception:
             sys.exc_clear()

testing/python/raises.py

@@ -118,3 +118,21 @@ def __call__(self):
         for o in gc.get_objects():
             assert type(o) is not T
 
+
+    def test_raises_match(self):
+        msg = r"with base \d+"
+        with pytest.raises(ValueError, match=msg):
+            int('asdf')
+
+        msg = "with base 10"
+        with pytest.raises(ValueError, match=msg):
+            int('asdf')
+
+        msg = "with base 16"
+        try:
+            with pytest.raises(ValueError, match=msg):
+                int('asdf', base=10)
+        except pytest.raises.Exception as e:
+            assert e.msg == "DID NOT RAISE {0} matching '{1}'".format(repr(ValueError), msg)
+        else:
+            assert False, "Expected pytest.raises.Exception"

testing/test_recwarn.py

@@ -112,10 +112,9 @@ def test_deprecated_explicit_call(self):
         pytest.deprecated_call(self.dep_explicit, 0)
 
     def test_deprecated_call_as_context_manager_no_warning(self):
-        with pytest.raises(pytest.fail.Exception) as ex:
+        with pytest.raises(pytest.fail.Exception, matches='^DID NOT WARN') as ex:
             with pytest.deprecated_call():
                 self.dep(1)
-        assert str(ex.value).startswith("DID NOT WARN")
 
     def test_deprecated_call_as_context_manager(self):
         with pytest.deprecated_call():