Catch exceptions in formatExceptionOnly.

Getting an exception when reporting about a different exception is not helpful.
On Python 3.11 this is needed for some HTTPErrors.

3 files changed, 53 insertions, 1 deletions

diff --git a/CHANGES.rst b/CHANGES.rst
index 88f6e1a..5d3521b 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -5,6 +5,10 @@
 4.6 (unreleased)
 ================
 
+- Catch exceptions in ``formatExceptionOnly``.
+  Getting an exception when reporting about a different exception is not helpful.
+  On Python 3.11 this is needed for some HTTPErrors.
+
 - Add official support for Python 3.11.
 
 
diff --git a/src/zope/exceptions/exceptionformatter.py b/src/zope/exceptions/exceptionformatter.py
index e6b0314..b61026c 100644
--- a/src/zope/exceptions/exceptionformatter.py
+++ b/src/zope/exceptions/exceptionformatter.py
@@ -171,7 +171,14 @@ class TextExceptionFormatter(object):
         return self.line_sep.join(result)
 
     def formatExceptionOnly(self, etype, value):
-        result = ''.join(traceback.format_exception_only(etype, value))
+        # We don't want to get an error when we format an error, so we
+        # compensate in our code.  For example, on Python 3.11.0 HTTPError
+        # gives an unhelpful KeyError in tempfile when Python formats it.
+        # See https://github.com/python/cpython/issues/90113
+        try:
+            result = ''.join(traceback.format_exception_only(etype, value))
+        except Exception:
+            result = str(value)
         return result
 
     def formatLastLine(self, exc_line):
diff --git a/src/zope/exceptions/tests/test_exceptionformatter.py b/src/zope/exceptions/tests/test_exceptionformatter.py
index 46eea6d..4d6c6fc 100644
--- a/src/zope/exceptions/tests/test_exceptionformatter.py
+++ b/src/zope/exceptions/tests/test_exceptionformatter.py
@@ -278,6 +278,47 @@ class TextExceptionFormatterTests(unittest.TestCase):
                          ''.join(
                              traceback.format_exception_only(ValueError, err)))
 
+    def test_formatExceptionOnly_httperror(self):
+        # On Python 3.11.0 HTTPError may behave wrongly, giving a KeyError in
+        # tempfile when Python tries to format it.
+        # See https://github.com/python/cpython/issues/90113
+        # or examples in Plone tests, especially doctests:
+        # https://github.com/plone/Products.CMFPlone/issues/3663
+        # We don't want to get an error when we format an error,
+        # so let's compensate in our code.
+        try:
+            from urllib.error import HTTPError
+        except ImportError:
+            # BBB for Python 2.7
+            from urllib2 import HTTPError
+        fmt = self._makeOne()
+        err = HTTPError('url', 400, 'oops', [], None)
+        result = fmt.formatExceptionOnly(HTTPError, err).strip()
+        # The output can differ too much per Python version,
+        # but it is just one line when stripped.
+        self.assertIn("400", result)
+        self.assertIn("oops", result)
+        self.assertIn("Error", result)
+        self.assertEqual(len(result.splitlines()), 1)
+
+    def test_formatException_httperror(self):
+        # See test_formatExceptionOnly_httperror.
+        # Here we check that formatException works.
+        try:
+            from urllib.error import HTTPError
+        except ImportError:
+            # BBB for Python 2.7
+            from urllib2 import HTTPError
+        fmt = self._makeOne()
+        err = HTTPError('url', 400, 'oops', [], None)
+        result = fmt.formatException(HTTPError, err, None)
+        self.assertEqual(result[0], 'Traceback (most recent call last):\n')
+        last = result[-1]
+        # The output can differ per Python version.
+        self.assertIn("400", last)
+        self.assertIn("oops", last)
+        self.assertIn("Error", last)
+
     def test_formatLastLine(self):
         fmt = self._makeOne()
         self.assertEqual(fmt.formatLastLine('XXX'), 'XXX')