Cap text diff computation for huge strings in non-verbose mode

changelog/12406.improvement.rst

@@ -0,0 +1,3 @@
+Failing string equality assertions on huge strings no longer take a very long time to report.
+
+At the default verbosity only a handful of diff lines are displayed, but the underlying diff was always computed over the full strings, which could take hours for multi-million-line texts. The diff is now computed over at most the first 100 lines of each string (after skipping any identical leading/trailing parts), with a message indicating that ``-v`` computes and shows the full diff.

src/_pytest/assertion/compare_text.py

@@ -9,6 +9,14 @@
 from _pytest.compat import assert_never
 
 
+# In non-verbose mode only a handful of diff lines are displayed, but
+# ``difflib.ndiff``'s runtime blows up on large inputs, so computing the
+# full diff can take hours only for almost all of it to be thrown away
+# (#12406). Diffing this many lines is near-instant while still producing
+# far more output than the display truncation will ever show.
+_MAX_NON_VERBOSE_DIFF_LINES = 100
+
+
 def _compare_eq_text(
     left: str,
     right: str,
@@ -75,13 +83,22 @@ def _diff_text(
         left = repr(str(left))
         right = repr(str(right))
         yield "Strings contain only whitespace, escaping them using repr()"
+    left_lines = left.splitlines(keepends)
+    right_lines = right.splitlines(keepends)
+    if verbose < 1 and (
+        len(left_lines) > _MAX_NON_VERBOSE_DIFF_LINES
+        or len(right_lines) > _MAX_NON_VERBOSE_DIFF_LINES
+    ):
+        yield (
+            f"Skipping all but the first {_MAX_NON_VERBOSE_DIFF_LINES} lines "
+            "of each text in diff, use -v to diff the full text"
+        )
+        left_lines = left_lines[:_MAX_NON_VERBOSE_DIFF_LINES]
+        right_lines = right_lines[:_MAX_NON_VERBOSE_DIFF_LINES]
     # "right" is the expected base against which we compare "left",
     # see https://github.com/pytest-dev/pytest/issues/3333
     yield from highlighter(
-        "\n".join(
-            line.strip("\n")
-            for line in ndiff(right.splitlines(keepends), left.splitlines(keepends))
-        ),
+        "\n".join(line.strip("\n") for line in ndiff(right_lines, left_lines)),
         lexer="diff",
     ).splitlines()
 

testing/test_assertion.py

@@ -522,6 +522,42 @@ def test_text_skipping_verbose(self) -> None:
         assert "- " + "a" * 50 + "eggs" in lines
         assert "+ " + "a" * 50 + "spam" in lines
 
+    def test_text_diff_huge_is_capped(self) -> None:
+        # Diffing large texts in full can take a very long time while only a
+        # few lines get displayed anyway, so the diff computation is capped
+        # in non-verbose mode (#12406).
+        left = "\n".join(f"spam {i}" for i in range(300))
+        right = "\n".join(f"eggs {i}" for i in range(300))
+        lines = callequal(left, right)
+        assert lines is not None
+        assert (
+            "Skipping all but the first 100 lines of each text in diff, "
+            "use -v to diff the full text" in lines
+        )
+        assert "+ spam 99" in lines
+        assert not any("spam 100" in line for line in lines)
+
+    def test_text_diff_huge_not_capped_verbose(self) -> None:
+        # With -v the full diff is computed, however long it takes.
+        left = "\n".join(f"spam {i}" for i in range(300))
+        right = "\n".join(f"eggs {i}" for i in range(300))
+        lines = callequal(left, right, verbose=1)
+        assert lines is not None
+        assert "+ spam 299" in lines
+        assert not any("Skipping" in line for line in lines)
+
+    def test_text_diff_huge_cap_after_leading_skip(self) -> None:
+        # A huge identical prefix is skipped first; the line cap then
+        # applies to the remaining differing region.
+        common = "\n".join(f"common {i}" for i in range(200))
+        left = common + "\n" + "\n".join(f"spam {i}" for i in range(300))
+        right = common + "\n" + "\n".join(f"eggs {i}" for i in range(300))
+        lines = callequal(left, right)
+        assert lines is not None
+        assert any("identical leading characters" in line for line in lines)
+        assert any("Skipping all but the first 100 lines" in line for line in lines)
+        assert not any("spam 150" in line for line in lines)
+
     def test_multiline_text_diff(self) -> None:
         left = "foo\nspam\nbar"
         right = "foo\neggs\nbar"