Reverse / fix meaning of "+/-" in error diffs

The convention is "assert result is expected".  Pytest's error diffs now
reflect this. "-" means that sth. expected is missing in the result and
"+" means that there are unexpected extras in the result.

Fixes: #3333

AUTHORS

@@ -246,6 +246,7 @@ Simon Gomizelj
 Skylar Downes
 Srinivas Reddy Thatiparthy
 Stefan Farmbauer
+Stefan Scherfke
 Stefan Zimmermann
 Stefano Taschini
 Steffen Allner

changelog/6673.breaking.rst

@@ -0,0 +1 @@
+Reversed / fix meaning of "+/-" in error diffs.  "-" means that sth. expected is missing in the result and "+" means that there are unexpected extras in the result.

doc/en/example/reportingdemo.rst

@@ -81,8 +81,8 @@ Here is a nice run of several failures and how ``pytest`` presents things:
         def test_eq_text(self):
     >       assert "spam" == "eggs"
     E       AssertionError: assert 'spam' == 'eggs'
-    E         - spam
-    E         + eggs
+    E         - eggs
+    E         + spam
 
     failure_demo.py:45: AssertionError
     _____________ TestSpecialisedExplanations.test_eq_similar_text _____________
@@ -92,9 +92,9 @@ Here is a nice run of several failures and how ``pytest`` presents things:
         def test_eq_similar_text(self):
     >       assert "foo 1 bar" == "foo 2 bar"
     E       AssertionError: assert 'foo 1 bar' == 'foo 2 bar'
-    E         - foo 1 bar
+    E         - foo 2 bar
     E         ?     ^
-    E         + foo 2 bar
+    E         + foo 1 bar
     E         ?     ^
 
     failure_demo.py:48: AssertionError
@@ -106,8 +106,8 @@ Here is a nice run of several failures and how ``pytest`` presents things:
     >       assert "foo\nspam\nbar" == "foo\neggs\nbar"
     E       AssertionError: assert 'foo\nspam\nbar' == 'foo\neggs\nbar'
     E           foo
-    E         - spam
-    E         + eggs
+    E         - eggs
+    E         + spam
     E           bar
 
     failure_demo.py:51: AssertionError
@@ -122,9 +122,9 @@ Here is a nice run of several failures and how ``pytest`` presents things:
     E       AssertionError: assert '111111111111...2222222222222' == '111111111111...2222222222222'
     E         Skipping 90 identical leading characters in diff, use -v to show
     E         Skipping 91 identical trailing characters in diff, use -v to show
-    E         - 1111111111a222222222
+    E         - 1111111111b222222222
     E         ?           ^
-    E         + 1111111111b222222222
+    E         + 1111111111a222222222
     E         ?           ^
 
     failure_demo.py:56: AssertionError

src/_pytest/assertion/util.py

@@ -225,9 +225,11 @@ def _diff_text(left: str, right: str, verbose: int = 0) -> List[str]:
         left = repr(str(left))
         right = repr(str(right))
         explanation += ["Strings contain only whitespace, escaping them using repr()"]
+    # "right" is the expected base against which we compare "left",
+    # see https://github.com/pytest-dev/pytest/issues/3333
     explanation += [
         line.strip("\n")
-        for line in ndiff(left.splitlines(keepends), right.splitlines(keepends))
+        for line in ndiff(right.splitlines(keepends), left.splitlines(keepends))
     ]
     return explanation
 
@@ -238,8 +240,8 @@ def _compare_eq_verbose(left: Any, right: Any) -> List[str]:
     right_lines = repr(right).splitlines(keepends)
 
     explanation = []  # type: List[str]
-    explanation += ["-" + line for line in left_lines]
-    explanation += ["+" + line for line in right_lines]
+    explanation += ["+" + line for line in left_lines]
+    explanation += ["-" + line for line in right_lines]
 
     return explanation
 
@@ -279,8 +281,10 @@ def _compare_eq_iterable(
         _surrounding_parens_on_own_lines(right_formatting)
 
     explanation = ["Full diff:"]
+    # "right" is the expected base against which we compare "left",
+    # see https://github.com/pytest-dev/pytest/issues/3333
     explanation.extend(
-        line.rstrip() for line in difflib.ndiff(left_formatting, right_formatting)
+        line.rstrip() for line in difflib.ndiff(right_formatting, left_formatting)
     )
     return explanation
 
@@ -315,8 +319,9 @@ def _compare_eq_sequence(
             break
 
     if comparing_bytes:
-        # when comparing bytes, it doesn't help to show the "sides contain one or more items"
-        # longer explanation, so skip it
+        # 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
@@ -443,7 +448,7 @@ def _notin_text(term: str, text: str, verbose: int = 0) -> List[str]:
     head = text[:index]
     tail = text[index + len(term) :]
     correct_text = head + tail
-    diff = _diff_text(correct_text, text, verbose)
+    diff = _diff_text(text, correct_text, verbose)
     newdiff = ["%s is contained here:" % saferepr(term, maxsize=42)]
     for line in diff:
         if line.startswith("Skipping"):

testing/acceptance_test.py

@@ -1277,8 +1277,8 @@ def test():
             "    def check():",
             ">       assert 1 == 2",
             "E       assert 1 == 2",
-            "E         -1",
-            "E         +2",
+            "E         +1",
+            "E         -2",
             "",
             "pdb.py:2: AssertionError",
             "*= 1 failed in *",