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 +1,3 @@
+Thresholds to trigger snippet truncation can now be set with :confval:`truncation_limit_lines` and :confval:`truncation_limit_chars`.
+
+See :ref:`truncation-params` for more information.

doc/en/how-to/output.rst

@@ -549,6 +549,28 @@ 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.
 
+
+.. _truncation-params:
+
+Modifying truncation limits
+--------------------------------------------------
+
+.. versionadded: 8.4
+
+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, whichever comes first.
+
+Setting both :confval:`truncation_limit_lines` and :confval:`truncation_limit_chars` to ``0`` will disable the truncation.
+However, setting only one of those values will disable one truncation mode, but will leave the other one intact.
+
 Creating resultlog format files
 --------------------------------------------------
 

doc/en/reference/reference.rst

@@ -1873,6 +1873,32 @@ passed multiple times. The expected format is ``name=value``. For example::
    Default: ``all``
 
 
+.. confval:: truncation_limit_chars
+
+    Controls the characters limit to truncate to.
+    Setting value to ``0`` disables the character limit for truncation.
+
+    .. code-block:: ini
+
+        [pytest]
+        truncation_limit_chars = 640
+
+   Default: ``640``
+
+
+.. confval:: truncation_limit_lines
+
+    Controls the lines limit to truncate to.
+    Setting value to ``0`` disables the lines limit for truncation.
+
+    .. code-block:: ini
+
+        [pytest]
+        truncation_limit_lines = 8
+
+   Default: ``8``
+
+
 .. confval:: usefixtures
 
     List of fixtures that will be applied to all test functions; this is semantically the same to apply

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=None,
+        help="Set threshold of LINES after which truncation will take effect",
+    )
+    parser.addini(
+        "truncation_limit_chars",
+        default=None,
+        help=("Set threshold of CHARS after which truncation will take effect"),
+    )
+
     Config._add_verbosity_ini(
         parser,
         Config.VERBOSITY_ASSERTIONS,

src/_pytest/assertion/truncate.py

@@ -12,41 +12,54 @@
 
 
 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)
+    should_truncate, max_lines, max_chars = _get_truncation_parameters(item)
+    if should_truncate:
+        return _truncate_explanation(
+            explanation,
+            max_lines=max_lines,
+            max_chars=max_chars,
+        )
     return explanation
 
 
-def _should_truncate_item(item: Item) -> bool:
-    """Whether or not this test item is eligible for truncation."""
+def _get_truncation_parameters(item: Item) -> tuple[bool, int, int]:
+    """Return the truncation parameters related to the given item, as (should truncate, max lines, max chars)."""
+    # We do not need to truncate if one of conditions is met:
+    # 1. Verbosity level is 2 or more;
+    # 2. Test is being run in CI environment;
+    # 3. Both truncation_limit_lines and truncation_limit_chars
+    #    .ini parameters are set to 0 explicitly.
+    max_lines = item.config.getini("truncation_limit_lines")
+    max_lines = int(max_lines if max_lines is not None else DEFAULT_MAX_LINES)
+
+    max_chars = item.config.getini("truncation_limit_chars")
+    max_chars = int(max_chars if max_chars is not None else DEFAULT_MAX_CHARS)
+
     verbose = item.config.get_verbosity(Config.VERBOSITY_ASSERTIONS)
-    return verbose < 2 and not util.running_on_ci()
+
+    should_truncate = verbose < 2 and not util.running_on_ci()
+    should_truncate = should_truncate and (max_lines > 0 or max_chars > 0)
+
+    return should_truncate, max_lines, max_chars
 
 
 def _truncate_explanation(
     input_lines: list[str],
-    max_lines: int | None = None,
-    max_chars: int | None = None,
+    max_lines: int,
+    max_chars: int,
 ) -> 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.
     """
-    if max_lines is None:
-        max_lines = DEFAULT_MAX_LINES
-    if max_chars is None:
-        max_chars = DEFAULT_MAX_CHARS
-
     # Check if truncation required
     input_char_count = len("".join(input_lines))
     # The length of the truncation explanation depends on the number of lines
@@ -71,16 +84,23 @@
     ):
         return input_lines
     # Truncate first to max_lines, and then truncate to max_chars if necessary
-    truncated_explanation = input_lines[:max_lines]
+    if max_lines > 0:
+        truncated_explanation = input_lines[:max_lines]
+    else:
+        truncated_explanation = input_lines
     truncated_char = True
     # We reevaluate the need to truncate chars following removal of some lines
-    if len("".join(truncated_explanation)) > tolerable_max_chars:
+    if len("".join(truncated_explanation)) > tolerable_max_chars and max_chars > 0:
         truncated_explanation = _truncate_by_char_count(
             truncated_explanation, max_chars
         )
     else:
         truncated_char = False
 
+    if truncated_explanation == input_lines:
+        # No truncation happened, so we do not need to add any explanations
+        return truncated_explanation
+
     truncated_line_count = len(input_lines) - len(truncated_explanation)
     if truncated_explanation[-1]:
         # Add ellipsis and take into account part-truncated final line

testing/test_assertion.py

@@ -1435,6 +1435,66 @@ def test_many_lines():
         result = pytester.runpytest()
         result.stdout.fnmatch_lines(["* 6*"])
 
+    @pytest.mark.parametrize(
+        ["truncation_lines", "truncation_chars", "expected_lines_hidden"],
+        (
+            (3, None, 3),
+            (4, None, 0),
+            (0, None, 0),
+            (None, 8, 6),
+            (None, 9, 0),
+            (None, 0, 0),
+            (0, 0, 0),
+            (0, 1000, 0),
+            (1000, 0, 0),
+        ),
+    )
+    def test_truncation_with_ini(
+        self,
+        monkeypatch,
+        pytester: Pytester,
+        truncation_lines: int | None,
+        truncation_chars: int | None,
+        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)
+
+        ini = "[pytest]\n"
+        if truncation_lines is not None:
+            ini += f"truncation_limit_lines = {truncation_lines}\n"
+        if truncation_chars is not None:
+            ini += f"truncation_limit_chars = {truncation_chars}\n"
+        pytester.makeini(ini)
+
+        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(