Remove newlines from left/right operands with '-vv' (#9743)

The left/right operands produced when `verbose > 1` should not contain newlines, because they are used to 
build the `summary` string. The `assertrepr_compare` function returns a list of lines, and the summary is one of those lines and should not contain newlines itself. 

Fix #9742

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Bruno Oliveira <nicoddemus@gmail.com>

AUTHORS

@@ -289,6 +289,7 @@ Ruaridh Williamson
 Russel Winder
 Ryan Wooden
 Saiprasad Kale
+Samuel Colvin
 Samuel Dion-Girardeau
 Samuel Searles-Bryant
 Samuele Pedroni

changelog/9742.improvement.rst

@@ -0,0 +1 @@
+Display assertion message without escaped newline characters with ``-vv``.

src/_pytest/_io/saferepr.py

@@ -107,6 +107,23 @@ def saferepr(obj: object, maxsize: Optional[int] = DEFAULT_REPR_MAX_SIZE) -> str
     return SafeRepr(maxsize).repr(obj)
 
 
+def saferepr_unlimited(obj: object) -> str:
+    """Return an unlimited-size safe repr-string for the given object.
+
+    As with saferepr, failing __repr__ functions of user instances
+    will be represented with a short exception info.
+
+    This function is a wrapper around simple repr.
+
+    Note: a cleaner solution would be to alter ``saferepr``this way
+    when maxsize=None, but that might affect some other code.
+    """
+    try:
+        return repr(obj)
+    except Exception as exc:
+        return _format_repr_exception(exc, obj)
+
+
 class AlwaysDispatchingPrettyPrinter(pprint.PrettyPrinter):
     """PrettyPrinter that always dispatches (regardless of width)."""
 

src/_pytest/assertion/util.py

@@ -14,8 +14,8 @@ from typing import Sequence
 import _pytest._code
 from _pytest import outcomes
 from _pytest._io.saferepr import _pformat_dispatch
-from _pytest._io.saferepr import safeformat
 from _pytest._io.saferepr import saferepr
+from _pytest._io.saferepr import saferepr_unlimited
 from _pytest.config import Config
 
 # The _reprcompare attribute on the util module is used by the new assertion
@@ -160,8 +160,8 @@ def assertrepr_compare(config, op: str, left: Any, right: Any) -> Optional[List[
     """Return specialised explanations for some operators/operands."""
     verbose = config.getoption("verbose")
     if verbose > 1:
-        left_repr = safeformat(left)
-        right_repr = safeformat(right)
+        left_repr = saferepr_unlimited(left)
+        right_repr = saferepr_unlimited(right)
     else:
         # XXX: "15 chars indentation" is wrong
         #      ("E       AssertionError: assert "); should use term width.

testing/io/test_saferepr.py

@@ -2,6 +2,7 @@ import pytest
 from _pytest._io.saferepr import _pformat_dispatch
 from _pytest._io.saferepr import DEFAULT_REPR_MAX_SIZE
 from _pytest._io.saferepr import saferepr
+from _pytest._io.saferepr import saferepr_unlimited
 
 
 def test_simple_repr():
@@ -179,3 +180,23 @@ def test_broken_getattribute():
     assert saferepr(SomeClass()).startswith(
         "<[RuntimeError() raised in repr()] SomeClass object at 0x"
     )
+
+
+def test_saferepr_unlimited():
+    dict5 = {f"v{i}": i for i in range(5)}
+    assert saferepr_unlimited(dict5) == "{'v0': 0, 'v1': 1, 'v2': 2, 'v3': 3, 'v4': 4}"
+
+    dict_long = {f"v{i}": i for i in range(1_000)}
+    r = saferepr_unlimited(dict_long)
+    assert "..." not in r
+    assert "\n" not in r
+
+
+def test_saferepr_unlimited_exc():
+    class A:
+        def __repr__(self):
+            raise ValueError(42)
+
+    assert saferepr_unlimited(A()).startswith(
+        "<[ValueError(42) raised in repr()] A object at 0x"
+    )

testing/test_assertion.py

@@ -1695,3 +1695,18 @@ def test_assertion_location_with_coverage(pytester: Pytester) -> None:
             "*= 1 failed in*",
         ]
     )
+
+
+def test_reprcompare_verbose_long() -> None:
+    a = {f"v{i}": i for i in range(11)}
+    b = a.copy()
+    b["v2"] += 10
+    lines = callop("==", a, b, verbose=2)
+    assert lines is not None
+    assert lines[0] == (
+        "{'v0': 0, 'v1': 1, 'v2': 2, 'v3': 3, 'v4': 4, 'v5': 5, "
+        "'v6': 6, 'v7': 7, 'v8': 8, 'v9': 9, 'v10': 10}"
+        " == "
+        "{'v0': 0, 'v1': 1, 'v2': 12, 'v3': 3, 'v4': 4, 'v5': 5, "
+        "'v6': 6, 'v7': 7, 'v8': 8, 'v9': 9, 'v10': 10}"
+    )