bdd: Improve output when comparing sessions

See #1440.

tests/integration/features/conftest.py

@@ -28,7 +28,6 @@ import collections
 import textwrap
 
 import pytest
-import yaml
 import pytest_bdd as bdd
 
 from helpers import utils
@@ -315,14 +314,7 @@ def compare_session(quteproc, expected):
     partial_compare is used, which means only the keys/values listed will be
     compared.
     """
-    # Translate ... to ellipsis in YAML.
-    loader = yaml.SafeLoader(expected)
-    loader.add_constructor('!ellipsis', lambda loader, node: ...)
-    loader.add_implicit_resolver('!ellipsis', re.compile(r'\.\.\.'), None)
-
-    data = quteproc.get_session()
-    expected = loader.get_data()
-    assert utils.partial_compare(data, expected)
+    quteproc.compare_session(expected)
 
 
 @bdd.then("no crash should happen")

tests/integration/quteprocess.py

@@ -419,7 +419,7 @@ class QuteProc(testprocess.Process):
             with open(session, encoding='utf-8') as f:
                 data = f.read()
 
-        self._log(data)
+        self._log('\nCurrent session data:\n' + data)
         return yaml.load(data)
 
     def get_content(self, plain=True):
@@ -464,6 +464,26 @@ class QuteProc(testprocess.Process):
             raise ValueError('Invalid response from qutebrowser: {}'
                              .format(message))
 
+    def compare_session(self, expected):
+        """Compare the current sessions against the given template.
+
+        partial_compare is used, which means only the keys/values listed will
+        be compared.
+        """
+        __tracebackhide__ = True
+        # Translate ... to ellipsis in YAML.
+        loader = yaml.SafeLoader(expected)
+        loader.add_constructor('!ellipsis', lambda loader, node: ...)
+        loader.add_implicit_resolver('!ellipsis', re.compile(r'\.\.\.'), None)
+
+        data = self.get_session()
+        expected = loader.get_data()
+        outcome = testutils.partial_compare(data, expected)
+        if not outcome:
+            msg = "Session comparison failed: {}".format(outcome.error)
+            msg += '\nsee stdout for details'
+            pytest.fail(msg)
+
 
 def _xpath_escape(text):
     """Escape a string to be used in an XPath expression.