Add ini option "assert_truncate_level"

src/_pytest/assertion/truncate.py

@@ -24,8 +24,11 @@ def _should_truncate_item(item):
     """
     Whether or not this test item is eligible for truncation.
     """
+    level = item.config.getini("assert_truncate_level")
     verbose = item.config.option.verbose
-    return verbose < 2 and not _running_on_ci()
+    if level == "auto":
+        return verbose < 2 and not _running_on_ci()
+    return int(level) > verbose
 
 
 def _running_on_ci():

src/_pytest/terminal.py

@@ -242,6 +242,17 @@ def pytest_addoption(parser):
         default="progress",
     )
 
+    # Experimental.
+    parser.addini(
+        "assert_truncate_level",
+        help=(
+            "Truncate explanations of assertion failures?  "
+            '("auto" (when verbosity < 2, and not running on CI), '
+            "or minimum verbosity level to trigger it (i.e. 0 for no truncation)."
+        ),
+        default="auto",
+    )
+
 
 def pytest_configure(config):
     reporter = TerminalReporter(config, sys.stdout)
@@ -1032,6 +1043,7 @@ def short_test_summary(self) -> None:
         if not self.reportchars:
             return
 
+        # NOTE: there's also _pytest.assertion._running_on_ci.
         if os.environ.get("CI") == "true" or not self.isatty:
             termwidth = None
         else:

testing/test_assertion.py

@@ -946,6 +946,16 @@ def test_many_lines():
             ]
         )
 
+        # test assert_truncate_level ini option.
+        result = testdir.runpytest("-o", "assert_truncate_level=1")
+        result.stdout.fnmatch_lines(
+            ["E         ...Full output truncated (2 lines hidden), use '-vv' to show"]
+        )
+        result = testdir.runpytest("-o", "assert_truncate_level=0")
+        result.stdout.fnmatch_lines(["* 6*"])
+        result = testdir.runpytest("-v", "-o", "assert_truncate_level=0")
+        result.stdout.fnmatch_lines(["* 6*"])
+
         result = testdir.runpytest("-vv")
         result.stdout.fnmatch_lines(["* 6*"])