Separate the various parts of the error report with newlines (#11659)

Previously the error report would have all sections glued together:

- The assertion representation
- The error explanation
- The full diff

This makes it hard to see at a glance where which one starts and ends.

One of the representation (dataclasses, tuples, attrs) does display a
newlines at the start already.

Let's add a newlines before the error explanation and before the full
diff, so we get an easier to read report.

This has one disadvantage: we get one line less in the least verbose
mode, where the output gets truncated.

changelog/11520.improvement.rst

@@ -1 +1,3 @@
 Improved very verbose diff output to color it as a diff instead of only red.
+
+Improved the error reporting to better separate each section.

src/_pytest/assertion/util.py

@@ -230,6 +230,8 @@ def assertrepr_compare(
     if not explanation:
         return None
 
+    if explanation[0] != "":
+        explanation = [""] + explanation
     return [summary] + explanation
 
 
@@ -332,7 +334,7 @@ def _compare_eq_iterable(
     left_formatting = PrettyPrinter().pformat(left).splitlines()
     right_formatting = PrettyPrinter().pformat(right).splitlines()
 
-    explanation = ["Full diff:"]
+    explanation = ["", "Full diff:"]
     # "right" is the expected base against which we compare "left",
     # see https://github.com/pytest-dev/pytest/issues/3333
     explanation.extend(

testing/python/approx.py

@@ -99,6 +99,7 @@ def test_error_messages_native_dtypes(self, assert_approx_raises_regex):
             2.0,
             1.0,
             [
+                "",
                 "  comparison failed",
                 f"  Obtained: {SOME_FLOAT}",
                 f"  Expected: {SOME_FLOAT} ± {SOME_FLOAT}",
@@ -113,6 +114,7 @@ def test_error_messages_native_dtypes(self, assert_approx_raises_regex):
                 "c": 3000000.0,
             },
             [
+                r"",
                 r"  comparison failed. Mismatched elements: 2 / 3:",
                 rf"  Max absolute difference: {SOME_FLOAT}",
                 rf"  Max relative difference: {SOME_FLOAT}",
@@ -130,6 +132,7 @@ def test_error_messages_native_dtypes(self, assert_approx_raises_regex):
                 "c": None,
             },
             [
+                r"",
                 r"  comparison failed. Mismatched elements: 2 / 3:",
                 r"  Max absolute difference: -inf",
                 r"  Max relative difference: -inf",
@@ -143,6 +146,7 @@ def test_error_messages_native_dtypes(self, assert_approx_raises_regex):
             [1.0, 2.0, 3.0, 4.0],
             [1.0, 3.0, 3.0, 5.0],
             [
+                r"",
                 r"  comparison failed. Mismatched elements: 2 / 4:",
                 rf"  Max absolute difference: {SOME_FLOAT}",
                 rf"  Max relative difference: {SOME_FLOAT}",
@@ -156,6 +160,7 @@ def test_error_messages_native_dtypes(self, assert_approx_raises_regex):
             (1, 2.2, 4),
             (1, 3.2, 4),
             [
+                r"",
                 r"  comparison failed. Mismatched elements: 1 / 3:",
                 rf"  Max absolute difference: {SOME_FLOAT}",
                 rf"  Max relative difference: {SOME_FLOAT}",
@@ -169,6 +174,7 @@ def test_error_messages_native_dtypes(self, assert_approx_raises_regex):
             [0.0],
             [1.0],
             [
+                r"",
                 r"  comparison failed. Mismatched elements: 1 / 1:",
                 rf"  Max absolute difference: {SOME_FLOAT}",
                 r"  Max relative difference: inf",
@@ -187,6 +193,7 @@ def test_error_messages_numpy_dtypes(self, assert_approx_raises_regex):
             a,
             b,
             [
+                r"",
                 r"  comparison failed. Mismatched elements: 1 / 20:",
                 rf"  Max absolute difference: {SOME_FLOAT}",
                 rf"  Max relative difference: {SOME_FLOAT}",
@@ -209,6 +216,7 @@ def test_error_messages_numpy_dtypes(self, assert_approx_raises_regex):
                 ]
             ),
             [
+                r"",
                 r"  comparison failed. Mismatched elements: 3 / 8:",
                 rf"  Max absolute difference: {SOME_FLOAT}",
                 rf"  Max relative difference: {SOME_FLOAT}",
@@ -224,6 +232,7 @@ def test_error_messages_numpy_dtypes(self, assert_approx_raises_regex):
             np.array([0.0]),
             np.array([1.0]),
             [
+                r"",
                 r"  comparison failed. Mismatched elements: 1 / 1:",
                 rf"  Max absolute difference: {SOME_FLOAT}",
                 r"  Max relative difference: inf",
@@ -241,6 +250,7 @@ def test_error_messages_invalid_args(self, assert_approx_raises_regex):
         message = "\n".join(str(e.value).split("\n")[1:])
         assert message == "\n".join(
             [
+                "  ",
                 "  Impossible to compare arrays with different shapes.",
                 "  Shapes: (2, 1) and (2, 2)",
             ]
@@ -251,6 +261,7 @@ def test_error_messages_invalid_args(self, assert_approx_raises_regex):
         message = "\n".join(str(e.value).split("\n")[1:])
         assert message == "\n".join(
             [
+                "  ",
                 "  Impossible to compare lists with different sizes.",
                 "  Lengths: 2 and 3",
             ]
@@ -264,6 +275,7 @@ def test_error_messages_with_different_verbosity(self, assert_approx_raises_rege
                 2.0,
                 1.0,
                 [
+                    "",
                     "  comparison failed",
                     f"  Obtained: {SOME_FLOAT}",
                     f"  Expected: {SOME_FLOAT} ± {SOME_FLOAT}",
@@ -277,15 +289,15 @@ def test_error_messages_with_different_verbosity(self, assert_approx_raises_rege
             a,
             b,
             [
-                r"  comparison failed. Mismatched elements: 20 / 20:",
-                rf"  Max absolute difference: {SOME_FLOAT}",
-                rf"  Max relative difference: {SOME_FLOAT}",
-                r"  Index \| Obtained\s+\| Expected",
-                rf"  \(0,\)\s+\| {SOME_FLOAT} \| {SOME_FLOAT} ± {SOME_FLOAT}",
-                rf"  \(1,\)\s+\| {SOME_FLOAT} \| {SOME_FLOAT} ± {SOME_FLOAT}",
-                rf"  \(2,\)\s+\| {SOME_FLOAT} \| {SOME_FLOAT} ± {SOME_FLOAT}...",
-                "",
-                rf"\s*...Full output truncated \({SOME_INT} lines hidden\), use '-vv' to show",
+                r"^  $",
+                r"^  comparison failed. Mismatched elements: 20 / 20:$",
+                rf"^  Max absolute difference: {SOME_FLOAT}$",
+                rf"^  Max relative difference: {SOME_FLOAT}$",
+                r"^  Index \| Obtained\s+\| Expected\s+$",
+                rf"^  \(0,\)\s+\| {SOME_FLOAT} \| {SOME_FLOAT} ± {SOME_FLOAT}e-{SOME_INT}$",
+                rf"^  \(1,\)\s+\| {SOME_FLOAT} \| {SOME_FLOAT} ± {SOME_FLOAT}e-{SOME_INT}\.\.\.$",
+                "^  $",
+                rf"^  ...Full output truncated \({SOME_INT} lines hidden\), use '-vv' to show$",
             ],
             verbosity_level=0,
         )
@@ -294,6 +306,7 @@ def test_error_messages_with_different_verbosity(self, assert_approx_raises_rege
             a,
             b,
             [
+                r"  ",
                 r"  comparison failed. Mismatched elements: 20 / 20:",
                 rf"  Max absolute difference: {SOME_FLOAT}",
                 rf"  Max relative difference: {SOME_FLOAT}",
@@ -652,6 +665,7 @@ def test_dict_for_div_by_zero(self, assert_approx_raises_regex):
             {"foo": 42.0},
             {"foo": 0.0},
             [
+                r"",
                 r"  comparison failed. Mismatched elements: 1 / 1:",
                 rf"  Max absolute difference: {SOME_FLOAT}",
                 r"  Max relative difference: inf",