From 2708376bdf153f44872bfc08fd09acb61365ad15 Mon Sep 17 00:00:00 2001
From: Alex Hall <alex.mojaki@gmail.com>
Date: Fri, 26 Apr 2019 08:32:18 +0200
Subject: [PATCH] Fully readable failure from assert_output

---
 tests/utils.py | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/tests/utils.py b/tests/utils.py
index 06da56f..1979534 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -4,6 +4,7 @@
 import re
 import abc
 import inspect
+from pysnooper.third_party.six.moves import zip_longest
 
 from python_toolbox import caching
 
@@ -232,13 +233,18 @@ def assert_output(output, expected_entries, prefix=None):
             if not line.startswith(prefix):
                 raise OutputFailure(line)
 
-    for expected_entry, line in zip(expected_entries, lines):
-        if not expected_entry.check(line):
-            raise OutputFailure(line)
+    any_mismatch = False
+    result = ''
+    template = '\n{line!s:%s}   {expected_entry}  {arrow}' % max(map(len, lines))
+    for expected_entry, line in zip_longest(expected_entries, lines, fillvalue=""):
+        mismatch = not (expected_entry and expected_entry.check(line))
+        any_mismatch |= mismatch
+        arrow = '<===' * mismatch
+        result += template.format(**locals())
 
     if len(lines) != len(expected_entries):
-        raise OutputFailure(
-            'Output has {} lines, while we expect '
-            '{} lines.'.format(
+        result += '\nOutput has {} lines, while we expect {} lines.'.format(
                 len(lines), len(expected_entries))
-        )
+
+    if any_mismatch:
+        raise OutputFailure(result)