add assert_big_text_equal

for better output when large strings differ

IPython/nbconvert/exporters/tests/test_notebook.py

@@ -8,6 +8,8 b' import json'
 from .base import ExportersTestsBase
 from ..notebook import NotebookExporter
 
+from IPython.testing.tools import assert_big_text_equal
+
 class TestNotebookExporter(ExportersTestsBase):
     """Contains test functions for notebook.py"""
 
@@ -21,7 +23,7 b' class TestNotebookExporter(ExportersTestsBase):'
             file_contents = f.read()
         (output, resources) = self.exporter_class().from_filename(self._get_notebook())
         assert len(output) > 0
-        self.assertEqual(output, file_contents)
+        assert_big_text_equal(output, file_contents)
 
     def test_downgrade_3(self):
         exporter = self.exporter_class(nbformat_version=3)

IPython/testing/tools.py

@@ -465,3 +465,23 b" def help_all_output_test(subcommand=''):"
     nt.assert_in("Class parameters", out)
     return out, err
 
+def assert_big_text_equal(a, b, chunk_size=80):
+    """assert that large strings are equal
+
+    Zooms in on first chunk that differs,
+    to give better info than vanilla assertEqual for large text blobs.
+    """
+    for i in range(0, len(a), chunk_size):
+        chunk_a = a[i:i + chunk_size]
+        chunk_b = b[i:i + chunk_size]
+        nt.assert_equal(chunk_a, chunk_b, "[offset: %i]\n%r != \n%r" % (
+            i, chunk_a, chunk_b))
+
+    if len(a) > len(b):
+        nt.fail("Length doesn't match (%i > %i). Extra text:\n%r" % (
+            len(a), len(b), a[len(b):]
+        ))
+    elif len(a) < len(b):
+        nt.fail("Length doesn't match (%i < %i). Extra text:\n%r" % (
+            len(a), len(b), b[len(a):]
+        ))