Add AssertPrints context manager to check output from tests.

IPython/testing/tests/test_tools.py

@@ -17,6 +17,7 b' from __future__ import with_statement'
 
 import os
 import sys
+import unittest
 
 import nose.tools as nt
 
@@ -71,3 +72,19 b' def test_temp_pyfile():'
     with open(fname) as fh2:
         src2 = fh2.read()
     yield nt.assert_equal(src2, src)
+
+class TestAssertPrints(unittest.TestCase):
+    def test_passing(self):
+        with tt.AssertPrints("abc"):
+            print "abcd"
+            print "def"
+            print b"ghi"
+    
+    def test_failing(self):
+        def func():
+            with tt.AssertPrints("abc"):
+                print "acd"
+                print "def"
+                print b"ghi"
+        
+        self.assertRaises(AssertionError, func)

IPython/testing/tools.py

@@ -34,6 +34,7 b' import sys'
 import tempfile
 
 from contextlib import contextmanager
+from io import StringIO
 
 try:
     # These tools are used by parts of the runtime, so we make the nose
@@ -46,9 +47,9 b' except ImportError:'
 
 from IPython.config.loader import Config
 from IPython.utils.process import find_cmd, getoutputerror
-from IPython.utils.text import list_strings
-from IPython.utils.io import temp_pyfile
-from IPython.utils.py3compat import PY3
+from IPython.utils.text import list_strings, getdefaultencoding
+from IPython.utils.io import temp_pyfile, Tee
+from IPython.utils import py3compat
 
 from . import decorators as dec
 from . import skipdoctest
@@ -210,7 +211,7 b' def ipexec(fname, options=None):'
     _ip = get_ipython()
     test_dir = os.path.dirname(__file__)
 
-    ipython_cmd = find_cmd('ipython3' if PY3 else 'ipython')
+    ipython_cmd = find_cmd('ipython3' if py3compat.PY3 else 'ipython')
     # Absolute path for filename
     full_fname = os.path.join(test_dir, fname)
     full_cmd = '%s %s %s' % (ipython_cmd, cmdargs, full_fname)
@@ -324,6 +325,47 b' def check_pairs(func, pairs):'
         out = func(inp)
         assert out == expected, pair_fail_msg.format(name, inp, expected, out)
 
+if py3compat.PY3:
+    MyStringIO = StringIO
+else:
+    # In Python 2, stdout/stderr can have either bytes or unicode written to them,
+    # so we need a class that can handle both.
+    class MyStringIO(StringIO):
+        def write(self, s):
+            s = py3compat.cast_unicode(s, encoding=getdefaultencoding())
+            super(MyStringIO, self).write(s)
+
+notprinted_msg = """Did not find {0!r} in printed output (on {1}):
+{2!r}"""
+class AssertPrints(object):
+    """Context manager for testing that code prints certain text.
+    
+    Examples
+    --------
+    >>> with AssertPrints("abc"):
+    ...     print "abcd"
+    ...     print "def"
+    ... 
+    abcd
+    def
+    """
+    def __init__(self, s, channel='stdout'):
+        self.s = s
+        self.channel = channel
+    
+    def __enter__(self):
+        self.orig_stream = getattr(sys, self.channel)
+        self.buffer = MyStringIO()
+        self.tee = Tee(self.buffer, channel=self.channel)
+        setattr(sys, self.channel, self.tee)
+    
+    def __exit__(self, etype, value, traceback):
+        self.tee.flush()
+        setattr(sys, self.channel, self.orig_stream)
+        printed = self.buffer.getvalue()
+        assert self.s in printed, notprinted_msg.format(self.s, self.channel, printed)
+        return False
+
 @contextmanager
 def mute_warn():
     from IPython.utils import warn

IPython/utils/tests/test_path.py

@@ -29,7 +29,7 b' from nose import with_setup'
 import IPython
 from IPython.testing import decorators as dec
 from IPython.testing.decorators import skip_if_not_win32, skip_win32
-from IPython.testing.tools import make_tempfile
+from IPython.testing.tools import make_tempfile, AssertPrints
 from IPython.utils import path, io
 from IPython.utils import py3compat
 
@@ -404,13 +404,8 b' def test_not_writable_ipdir():'
     ipdir = os.path.join(tmpdir, '.ipython')
     os.mkdir(ipdir)
     os.chmod(ipdir, 600)
-    stderr = io.stderr
-    pipe = StringIO()
-    io.stderr = pipe
-    ipdir = path.get_ipython_dir()
-    io.stderr.flush()
-    io.stderr = stderr
-    nt.assert_true('WARNING' in pipe.getvalue())
+    with AssertPrints('WARNING', channel='stderr'):
+        ipdir = path.get_ipython_dir()
     env.pop('IPYTHON_DIR', None)
 
 def test_unquote_filename():