Improve comparison of byte strings (supersedes #5267)

changelog/5260.bugfix.rst

@@ -0,0 +1,17 @@
+Improved comparison of byte strings.
+
+When comparing bytes, the assertion message used to show the byte numeric value when showing the differences::
+
+        def test():
+    >       assert b'spam' == b'eggs'
+    E       AssertionError: assert b'spam' == b'eggs'
+    E         At index 0 diff: 115 != 101
+    E         Use -v to get the full diff
+
+It now shows the actual ascii representation instead, which is often more useful::
+
+        def test():
+    >       assert b'spam' == b'eggs'
+    E       AssertionError: assert b'spam' == b'eggs'
+    E         At index 0 diff: b's' != b'e'
+    E         Use -v to get the full diff

src/_pytest/assertion/util.py

@@ -254,17 +254,38 @@ def _compare_eq_iterable(left, right, verbose=0):
 
 
 def _compare_eq_sequence(left, right, verbose=0):
+    comparing_bytes = isinstance(left, bytes) and isinstance(right, bytes)
     explanation = []
     len_left = len(left)
     len_right = len(right)
     for i in range(min(len_left, len_right)):
         if left[i] != right[i]:
+            if comparing_bytes:
+                # when comparing bytes, we want to see their ascii representation
+                # instead of their numeric values (#5260)
+                # using a slice gives us the ascii representation:
+                # >>> s = b'foo'
+                # >>> s[0]
+                # 102
+                # >>> s[0:1]
+                # b'f'
+                left_value = left[i : i + 1]
+                right_value = right[i : i + 1]
+            else:
+                left_value = left[i]
+                right_value = right[i]
+
             explanation += [
-                "At index {} diff: {!r} != {!r}".format(i, left[i], right[i])
+                "At index {} diff: {!r} != {!r}".format(i, left_value, right_value)
             ]
             break
-    len_diff = len_left - len_right
 
+    if comparing_bytes:
+        # when comparing bytes, it doesn't help to show the "sides contain one or more items"
+        # longer explanation, so skip it
+        return explanation
+
+    len_diff = len_left - len_right
     if len_diff:
         if len_diff > 0:
             dir_with_more = "Left"

testing/test_assertion.py

@@ -331,6 +331,27 @@ def test_multiline_text_diff(self):
         assert "- spam" in diff
         assert "+ eggs" in diff
 
+    def test_bytes_diff_normal(self):
+        """Check special handling for bytes diff (#5260)"""
+        diff = callequal(b"spam", b"eggs")
+
+        assert diff == [
+            "b'spam' == b'eggs'",
+            "At index 0 diff: b's' != b'e'",
+            "Use -v to get the full diff",
+        ]
+
+    def test_bytes_diff_verbose(self):
+        """Check special handling for bytes diff (#5260)"""
+        diff = callequal(b"spam", b"eggs", verbose=True)
+        assert diff == [
+            "b'spam' == b'eggs'",
+            "At index 0 diff: b's' != b'e'",
+            "Full diff:",
+            "- b'spam'",
+            "+ b'eggs'",
+        ]
+
     def test_list(self):
         expl = callequal([0, 1], [0, 2])
         assert len(expl) > 1