Output actual character differences when tests fail

test.py

@@ -7,11 +7,14 @@
 
 from __future__ import with_statement
 
-from hypothesis import given, assume, Settings, Verbosity
-import hypothesis.strategies as st
+import textwrap
+from difflib import ndiff
+from io import open
 from os import listdir
 from os.path import dirname, isdir, join, realpath, relpath, splitext
 
+import hypothesis.strategies as st
+from hypothesis import given, assume, Settings, Verbosity
 from nose.tools import eq_, assert_raises
 
 import chardet
@@ -27,12 +30,28 @@
 def check_file_encoding(file_name, encoding):
     """ Ensure that we detect the encoding for file_name correctly. """
     with open(file_name, 'rb') as f:
-        result = chardet.detect(f.read())
+        input_bytes = f.read()
+        result = chardet.detect(input_bytes)
+        try:
+            expected_unicode = input_bytes.decode(encoding)
+        except LookupError:
+            expected_unicode = ''
+        try:
+            detected_unicode = input_bytes.decode(result['encoding'])
+        except (LookupError, UnicodeDecodeError):
+            detected_unicode = ''
     encoding = EQUIVALENT_ENCODINGS.get(encoding, encoding)
-    eq_(result['encoding'].lower(), encoding, ("Expected %s, but got %s for "
-                                               "%s" % (encoding,
-                                                       result['encoding'],
-                                                       file_name)))
+    if result['encoding'].lower() != encoding:
+        wrapped_expected = '\n'.join(textwrap.wrap(expected_unicode, 100)) + '\n'
+        wrapped_detected = '\n'.join(textwrap.wrap(detected_unicode, 100)) + '\n'
+        diff = ''.join(ndiff(wrapped_expected.splitlines(keepends=True),
+                             wrapped_detected.splitlines(keepends=True)))
+    else:
+        diff = ''
+    eq_(result['encoding'].lower(), encoding,
+        ("Expected %s, but got %s for %s.  Character differences: \n%s" %
+         (encoding, result['encoding'], file_name, diff)))
+
 
 
 def test_encoding_detection():