Show stdout and stderr for linter bears

Print the `stdout` and `stderr` from linter bears when tests
fail which is helpful for debugging.

Closes coala/coala-bears#455

coalib/testing/LocalBearTestHelper.py

@@ -1,6 +1,7 @@
 import queue
 import unittest
-from contextlib import contextmanager
+from contextlib import contextmanager, ExitStack
+from unittest.mock import patch
 
 import pytest
 
@@ -14,14 +15,38 @@
 @contextmanager
 def execute_bear(bear, *args, **kwargs):
     try:
-        bear_output_generator = bear.execute(*args, **kwargs)
+        console_output = []
+
+        # For linters provide additional information, such as
+        # stdout and stderr.
+        with ExitStack() as stack:
+            if hasattr(bear, 'process_output'):
+                console_output.append('The program yielded '
+                                      'the following output:\n')
+                old_process_output = bear.process_output
+
+                def new_process_output(output, filename=None, file=None):
+                    if isinstance(output, tuple):
+                        stdout, stderr = output
+                        console_output.append('Stdout:\n' + stdout)
+                        console_output.append('Stderr:\n' + stderr)
+                    else:
+                        console_output.append(output)
+                    return old_process_output(output, filename, file)
+
+                stack.enter_context(patch.object(
+                    bear, 'process_output', wraps=new_process_output))
+
+            bear_output_generator = bear.execute(*args, **kwargs)
+
         assert bear_output_generator is not None, \
             'Bear returned None on execution\n'
         yield bear_output_generator
     except Exception as err:
         msg = []
         while not bear.message_queue.empty():
             msg.append(bear.message_queue.get().message)
+        msg += console_output
         raise AssertionError(str(err) + ''.join('\n' + m for m in msg))
     return list(bear_output_generator)
 

tests/testing/LocalBearTestHelperTest.py

@@ -1,10 +1,12 @@
 from queue import Queue
+import sys
 import unittest
 
 from tests.test_bears.TestBear import TestBear
 from tests.test_bears.TestBearDep import (TestDepBearBDependsA,
                                           TestDepBearCDependsB,
                                           TestDepBearDependsAAndAA)
+from coalib.bearlib.abstractions.Linter import linter
 from coalib.settings.Section import Section
 from coalib.settings.Setting import Setting
 from coalib.testing.LocalBearTestHelper import verify_local_bear, execute_bear
@@ -101,6 +103,45 @@ def setUp(self):
         section.append(Setting('exception', True))
         self.uut = TestBear(section, Queue())
 
+    def test_stdout_stderr_on_linter_test_fail(self):
+
+        class TestLinter:
+            @staticmethod
+            def process_output(output, filename, file):
+                pass
+
+            @staticmethod
+            def create_arguments(filename, file, config_file):
+                code = '\n'.join(['import sys',
+                                  "print('hello stdout')",
+                                  "print('hello stderr', file=sys.stderr)"])
+                return '-c', code
+
+        # Testing with both stdout and stderr enabled
+        uut = (linter(sys.executable, use_stdout=True, use_stderr=True)
+               (TestLinter)
+               (Section('TEST_SECTION'), Queue()))
+        try:
+            with execute_bear(uut, 'filename', ['file']) as result:
+                raise AssertionError
+        except AssertionError as ex:
+            self.assertIn('The program yielded the following output:', str(ex))
+            self.assertIn('Stdout:', str(ex))
+            self.assertIn('hello stdout', str(ex))
+            self.assertIn('Stderr:', str(ex))
+            self.assertIn('hello stderr', str(ex))
+
+        # Testing with only stdout enabled
+        uut = (linter(sys.executable, use_stdout=True)
+               (TestLinter)
+               (Section('TEST_SECTION'), Queue()))
+        try:
+            with execute_bear(uut, 'filename', ['file']) as result:
+                raise AssertionError
+        except AssertionError as ex:
+            self.assertIn('The program yielded the following output:', str(ex))
+            self.assertIn('hello stdout', str(ex))
+
     def test_exception(self):
 
         with self.assertRaises(AssertionError), execute_bear(