always show full diff in ci

changelog/9023.feature.rst

@@ -0,0 +1,4 @@
+
+Full diffs are now always shown for equality assertions of iterables when
+`CI` or ``BUILD_NUMBER`` is found in the environment, even when ``-v`` isn't
+used.

src/_pytest/assertion/truncate.py

@@ -3,10 +3,10 @@
 Current default behaviour is to truncate assertion explanations at
 ~8 terminal lines, unless running in "-vv" mode or running on CI.
 """
-import os
 from typing import List
 from typing import Optional
 
+from _pytest.assertion import util
 from _pytest.nodes import Item
 
 
@@ -27,13 +27,7 @@ def truncate_if_required(
 def _should_truncate_item(item: Item) -> bool:
     """Whether or not this test item is eligible for truncation."""
     verbose = item.config.option.verbose
-    return verbose < 2 and not _running_on_ci()
-
-
-def _running_on_ci() -> bool:
-    """Check if we're currently running on a CI system."""
-    env_vars = ["CI", "BUILD_NUMBER"]
-    return any(var in os.environ for var in env_vars)
+    return verbose < 2 and not util.running_on_ci()
 
 
 def _truncate_explanation(

src/_pytest/assertion/util.py

@@ -1,5 +1,6 @@
 """Utilities for assertion debugging."""
 import collections.abc
+import os
 import pprint
 from typing import AbstractSet
 from typing import Any
@@ -17,7 +18,6 @@
 from _pytest._io.saferepr import saferepr
 from _pytest.config import Config
 
-
 # The _reprcompare attribute on the util module is used by the new assertion
 # interpretation code and assertion rewriter to detect this plugin was
 # loaded and in turn call the hooks defined here as part of the
@@ -287,7 +287,7 @@ def _surrounding_parens_on_own_lines(lines: List[str]) -> None:
 def _compare_eq_iterable(
     left: Iterable[Any], right: Iterable[Any], verbose: int = 0
 ) -> List[str]:
-    if not verbose:
+    if not verbose and not running_on_ci():
         return ["Use -v to get the full diff"]
     # dynamic import to speedup pytest
     import difflib
@@ -490,3 +490,9 @@ def _notin_text(term: str, text: str, verbose: int = 0) -> List[str]:
         else:
             newdiff.append(line)
     return newdiff
+
+
+def running_on_ci() -> bool:
+    """Check if we're currently running on a CI system."""
+    env_vars = ["CI", "BUILD_NUMBER"]
+    return any(var in os.environ for var in env_vars)

testing/test_assertion.py

@@ -13,6 +13,7 @@
 from _pytest import outcomes
 from _pytest.assertion import truncate
 from _pytest.assertion import util
+from _pytest.monkeypatch import MonkeyPatch
 from _pytest.pytester import Pytester
 
 
@@ -448,6 +449,25 @@ def test_iterable_full_diff(self, left, right, expected) -> None:
         assert verbose_expl is not None
         assert "\n".join(verbose_expl).endswith(textwrap.dedent(expected).strip())
 
+    def test_iterable_full_diff_ci(
+        self, monkeypatch: MonkeyPatch, pytester: Pytester
+    ) -> None:
+        pytester.makepyfile(
+            r"""
+            def test_full_diff():
+                left = [0, 1]
+                right = [0, 2]
+                assert left == right
+        """
+        )
+        monkeypatch.setenv("CI", "true")
+        result = pytester.runpytest()
+        result.stdout.fnmatch_lines(["E         Full diff:"])
+
+        monkeypatch.delenv("CI", raising=False)
+        result = pytester.runpytest()
+        result.stdout.fnmatch_lines(["E         Use -v to get the full diff"])
+
     def test_list_different_lengths(self) -> None:
         expl = callequal([0, 1], [0, 1, 2])
         assert expl is not None