Add testing function check_pairs to check input/output pairs against a function, and produce useful failure messages.

IPython/testing/tools.py

@@ -281,3 +281,29 b' class TempFileMixin(object):'
                 # delete it.  I have no clue why
                 pass
 
+pair_fail_msg = ("Testing function {0}\n\n"
+                "In:\n"
+                "  {1!r}\n"
+                "Expected:\n"
+                "  {2!r}\n"
+                "Got:\n"
+                "  {3!r}\n")
+def check_pairs(func, pairs):
+    """Utility function for the common case of checking a function with a 
+    sequence of input/output pairs.
+    
+    Parameters
+    ----------
+    func : callable
+      The function to be tested. Should accept a single argument.
+    pairs : iterable
+      A list of (input, expected_output) tuples.
+    
+    Returns
+    -------
+    None. Raises an AssertionError if any output does not match the expected
+    value.
+    """
+    for inp, expected in pairs:
+        out = func(inp)
+        assert out == expected, pair_fail_msg.format(func.func_name, inp, expected, out)