Introduce test.test_support.TransientResource. It's a context manager to

surround calls to resources that may or may not be available.  Specifying the
expected exception and attributes to be raised if the resource is not available
prevents overly broad catching of exceptions.

This is meant to help suppress spurious failures by raising
test.test_support.ResourceDenied if the exception matches.  It would probably
be good to go through the various network tests and surround the calls to catch
connection timeouts (as done with test_socket_ssl in this commit).

4 files changed, 39 insertions, 1 deletions

diff --git a/Doc/lib/libtest.tex b/Doc/lib/libtest.tex
index f30b49b..326c5f0 100644
--- a/Doc/lib/libtest.tex
+++ b/Doc/lib/libtest.tex
@@ -285,6 +285,14 @@ originated from.
 
 The \module{test.test_support} module defines the following classes:
 
+\begin{classdesc}{TransientResource}{exc\optional{, **kwargs}}
+Create a context manager that raises \class{ResourceDenied} if the specified
+exception type is raised.  Any keyword arguments are treated as name/value
+pairs to be compared against any exception raised with the \code{with}
+statement.  Only if all pairs match is \class{ResourceDenied} raised.
+\versionadded{2.6}
+\end{classdesc}
+
 \begin{classdesc}{EnvironmentVarGuard}{}
 Class used to temporarily set or unset environment variables.  Instances can be
 used as a context manager.
diff --git a/Lib/test/test_socket_ssl.py b/Lib/test/test_socket_ssl.py
index 3c9c9f0..8fb7e4c 100644
--- a/Lib/test/test_socket_ssl.py
+++ b/Lib/test/test_socket_ssl.py
@@ -27,7 +27,8 @@ def test_basic():
         print "didn't raise TypeError"
     socket.RAND_add("this is a random string", 75.0)
 
-    f = urllib.urlopen('https://sf.net')
+    with test_support.TransientResource(IOError, errno=errno.ETIMEDOUT):
+        f = urllib.urlopen('https://sf.net')
     buf = f.read()
     f.close()
 
diff --git a/Lib/test/test_support.py b/Lib/test/test_support.py
index 82fd5f4..2e47aed 100644
--- a/Lib/test/test_support.py
+++ b/Lib/test/test_support.py
@@ -312,6 +312,31 @@ class EnvironmentVarGuard(object):
         for unset in self._unset:
             del self._environ[unset]
 
+class TransientResource(object):
+
+    """Raise ResourceDenied if an exception is raised while the context manager
+    is in effect that matches the specified exception and attributes."""
+
+    def __init__(self, exc, **kwargs):
+        self.exc = exc
+        self.attrs = kwargs
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, type_=None, value=None, traceback=None):
+        """If type_ is a subclass of self.exc and value has attributes matching
+        self.attrs, raise ResourceDenied.  Otherwise let the exception
+        propagate (if any)."""
+        if type_ is not None and issubclass(self.exc, type_):
+            for attr, attr_value in self.attrs.iteritems():
+                if not hasattr(value, attr):
+                    break
+                if getattr(value, attr) != attr_value:
+                    break
+            else:
+                raise ResourceDenied("an optional resource is not available")
+
 
 #=======================================================================
 # Decorator for running a function in a different locale, correctly resetting
diff --git a/Misc/NEWS b/Misc/NEWS
index 0ceb078..b80dd32 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -516,6 +516,10 @@ Extension Modules
 Tests
 -----
 
+- Added test.test_support.TransientResource which is a context manager to
+  surround calls to resources that are not guaranteed to work even if
+  test.test_support.requires says that the resource should exist.
+
 - Added a test for slicing of an exception.
 
 - Added test.test_support.EnvironmentVarGuard.  It's a class that provides a