Implement CLI options to set truncation thresholds

AUTHORS

@@ -328,6 +328,7 @@ Paul Müller
 Paul Reece
 Pauli Virtanen
 Pavel Karateev
+Pavel Zhukov
 Paweł Adamczak
 Pedro Algarvio
 Petter Strandmark

changelog/12765.feature.rst

@@ -0,0 +1 @@
+Thresholds to trigger snippet truncation can now be set with `--truncation-limit-lines` and `--truncation-limit-chars` command line options.

doc/en/how-to/output.rst

@@ -549,6 +549,22 @@ captured output:
     By default, parametrized variants of skipped tests are grouped together if
     they share the same skip reason. You can use ``--no-fold-skipped`` to print each skipped test separately.
 
+
+Modifying truncation limits
+--------------------------------------------------
+
+Default truncation limits are 8 lines or 640 characters, whichever comes first.
+To set custom truncation limits you can use following ``pytest.ini`` file options:
+
+.. code-block:: ini
+
+    [pytest]
+    truncation_limit_lines = 10
+    truncation_limit_chars = 90
+
+That will cause pytest to truncate the assertions to 10 lines or 90 characters.
+
+
 Creating resultlog format files
 --------------------------------------------------
 

src/_pytest/assertion/__init__.py

@@ -45,6 +45,18 @@ def pytest_addoption(parser: Parser) -> None:
         help="Enables the pytest_assertion_pass hook. "
         "Make sure to delete any previously generated pyc cache files.",
     )
+
+    parser.addini(
+        "truncation_limit_lines",
+        default=0,
+        help="Set threshold of LINES after which truncation will take effect",
+    )
+    parser.addini(
+        "truncation_limit_chars",
+        default=0,
+        help=("Set threshold of CHARS after which truncation will take effect"),
+    )
+
     Config._add_verbosity_ini(
         parser,
         Config.VERBOSITY_ASSERTIONS,

src/_pytest/assertion/truncate.py

@@ -12,16 +12,24 @@
 
 
 DEFAULT_MAX_LINES = 8
-DEFAULT_MAX_CHARS = 8 * 80
+DEFAULT_MAX_CHARS = DEFAULT_MAX_LINES * 80
 USAGE_MSG = "use '-vv' to show"
 
 
-def truncate_if_required(
-    explanation: list[str], item: Item, max_length: int | None = None
-) -> list[str]:
+def truncate_if_required(explanation: list[str], item: Item) -> list[str]:
     """Truncate this assertion explanation if the given test item is eligible."""
     if _should_truncate_item(item):
-        return _truncate_explanation(explanation)
+        max_lines = int(
+            item.config.getini("truncation_limit_lines") or DEFAULT_MAX_LINES
+        )
+        max_chars = int(
+            item.config.getini("truncation_limit_chars") or DEFAULT_MAX_CHARS
+        )
+        return _truncate_explanation(
+            explanation,
+            max_lines=max_lines,
+            max_chars=max_chars,
+        )
     return explanation
 
 
@@ -38,7 +46,7 @@ def _truncate_explanation(
 ) -> list[str]:
     """Truncate given list of strings that makes up the assertion explanation.
 
-    Truncates to either 8 lines, or 640 characters - whichever the input reaches
+    Truncates to either max_lines, or max_chars - whichever the input reaches
     first, taking the truncation explanation into account. The remaining lines
     will be replaced by a usage message.
     """

testing/test_assertion.py

@@ -1435,6 +1435,61 @@ def test_many_lines():
         result = pytester.runpytest()
         result.stdout.fnmatch_lines(["* 6*"])
 
+    @pytest.mark.parametrize(
+        ["truncation_mode", "truncation_limit", "expected_lines_hidden"],
+        (
+            ("lines", 3, 3),
+            ("lines", 4, 0),
+            ("chars", 8, 6),
+            ("chars", 9, 0),
+        ),
+    )
+    def test_truncation_with_ini(
+        self,
+        monkeypatch,
+        pytester: Pytester,
+        truncation_mode: str,
+        truncation_limit: int,
+        expected_lines_hidden: int,
+    ) -> None:
+        pytester.makepyfile(
+            """\
+            string_a = "123456789\\n23456789\\n3"
+            string_b = "123456789\\n23456789\\n4"
+
+            def test():
+                assert string_a == string_b
+            """
+        )
+
+        # This test produces 6 lines of diff output or 79 characters
+        # So the effect should be when threshold is < 4 lines (considering 2 additional lines for explanation)
+        # Or < 9 characters (considering 70 additional characters for explanation)
+
+        monkeypatch.delenv("CI", raising=False)
+
+        pytester.makeini(
+            f"""
+            [pytest]
+            truncation_limit_{truncation_mode} = {truncation_limit}
+            """
+        )
+
+        result = pytester.runpytest()
+
+        if expected_lines_hidden != 0:
+            result.stdout.fnmatch_lines(
+                [f"*truncated ({expected_lines_hidden} lines hidden)*"]
+            )
+        else:
+            result.stdout.no_fnmatch_line("*truncated*")
+            result.stdout.fnmatch_lines(
+                [
+                    "*- 4*",
+                    "*+ 3*",
+                ]
+            )
+
 
 def test_python25_compile_issue257(pytester: Pytester) -> None:
     pytester.makepyfile(