Add support for attaching logs on error.

fixtures/_fixtures/logger.py

@@ -73,7 +73,8 @@ class FakeLogger(Fixture):
     """Replace a logger and capture its output."""
 
     def __init__(self, name="", level=INFO, format=None,
-                 datefmt=None, nuke_handlers=True, formatter=None):
+                 datefmt=None, nuke_handlers=True, formatter=None,
+                 only_on_error=False):
         """Create a FakeLogger fixture.
 
         :param name: The name of the logger to replace. Defaults to "".
@@ -86,6 +87,9 @@ def __init__(self, name="", level=INFO, format=None,
             existing messages going to e.g. stdout). Defaults to True.
         :param formatter: a custom log formatter class. Use this if you want
             to use a log Formatter other than the default one in python.
+        :param only_on_error: Only attach the captured output to the TestResult
+            if the test fails. This can be important for some test suites where
+            the full debug logging needed is enormous.
 
         Example:
 
@@ -101,10 +105,12 @@ def test_log(self)
         self._datefmt = datefmt
         self._nuke_handlers = nuke_handlers
         self._formatter = formatter
+        self._only_on_error = only_on_error
 
     def _setUp(self):
         name = _u("pythonlogging:'%s'") % self._name
-        output = self.useFixture(StringStream(name)).stream
+        output = self.useFixture(
+            StringStream(name, only_on_error=self._only_on_error)).stream
         self._output = output
         handler = StreamHandlerRaiseException(output)
         if self._format:

fixtures/_fixtures/streams.py

@@ -32,36 +32,48 @@ class Stream(Fixture):
     :attr stream: The file-like object.
     """
 
-    def __init__(self, detail_name, stream_factory):
+    def __init__(self, detail_name, stream_factory, only_on_error=False):
         """Create a ByteStream.
 
         :param detail_name: Use this as the name of the stream.
         :param stream_factory: Called to construct a pair of streams:
             (write_stream, content_stream).
+        :param only_on_error: Only attach the stream output if an error occurs.
         """
         self._detail_name = detail_name
         self._stream_factory = stream_factory
+        self._only_on_error = only_on_error
 
     def _setUp(self):
         write_stream, read_stream = self._stream_factory()
         self.stream = write_stream
+        self._read_stream = read_stream
+        if self._only_on_error:
+            self.addOnException(self._add_stream_detail)
+        else:
+            self._add_stream_detail()
+
+    def _add_stream_detail(self):
         self.addDetail(self._detail_name,
-            testtools.content.content_from_stream(read_stream, seek_offset=0))
+            testtools.content.content_from_stream(
+                self._read_stream, seek_offset=0))
 
 
 def _byte_stream_factory():
     result = io.BytesIO()
     return (result, result)
 
 
-def ByteStream(detail_name):
+def ByteStream(detail_name, only_on_error=only_on_error):
     """Provide a file-like object that accepts bytes and expose as a detail.
 
     :param detail_name: The name of the detail.
+    :param only_on_error: Only attach the stream output if an error occurs.
     :return: A fixture which has an attribute `stream` containing the file-like
         object.
     """
-    return Stream(detail_name, _byte_stream_factory)
+    return Stream(
+        detail_name, _byte_stream_factory, only_on_error=only_on_error)
 
 
 def _string_stream_factory():
@@ -81,14 +93,16 @@ def safe_write(str_or_bytes):
     return upper, lower
 
 
-def StringStream(detail_name):
+def StringStream(detail_name, only_on_error=False):
     """Provide a file-like object that accepts strings and expose as a detail.
 
     :param detail_name: The name of the detail.
+    :param only_on_error: Only attach the stream output if an error occurs.
     :return: A fixture which has an attribute `stream` containing the file-like
         object.
     """
-    return Stream(detail_name, _string_stream_factory)
+    return Stream(
+        detail_name, _string_stream_factory, only_on_error=only_on_error)
 
 
 def DetailStream(detail_name):