Truncate diff of very long texts if not in verbose mode (fix #12406)

AUTHORS

@@ -103,6 +103,7 @@ CrazyMerlyn
 Cristian Vera
 Cyrus Maden
 Damian Skrzypczak
+Daniel Diniz
 Daniel Grana
 Daniel Hahler
 Daniel Miller

changelog/12406.improvement.rst

@@ -0,0 +1,2 @@
+The ``_diff_text()`` function now truncates texts before diffing them
+if not in verbose mode.  -- by :user:`devdanzin`.

src/_pytest/assertion/util.py

@@ -21,6 +21,8 @@
 from _pytest._io.pprint import PrettyPrinter
 from _pytest._io.saferepr import saferepr
 from _pytest._io.saferepr import saferepr_unlimited
+from _pytest.assertion.truncate import DEFAULT_MAX_CHARS
+from _pytest.assertion.truncate import DEFAULT_MAX_LINES
 from _pytest.config import Config
 
 
@@ -285,17 +287,18 @@
     explanation: list[str] = []
 
     if verbose < 1:
-        i = 0  # just in case left or right has zero length
-        for i in range(min(len(left), len(right))):
-            if left[i] != right[i]:
+        leading_skipped = 0  # just in case left or right has zero length
+        for leading_skipped in range(min(len(left), len(right))):
+            if left[leading_skipped] != right[leading_skipped]:
                 break
-        if i > 42:
-            i -= 10  # Provide some context
+        if leading_skipped > 42:
+            leading_skipped -= 10  # Provide some context
             explanation = [
-                f"Skipping {i} identical leading characters in diff, use -v to show"
+                f"Skipping {leading_skipped} identical leading characters in diff, use -v to show"
             ]
-            left = left[i:]
-            right = right[i:]
+            left = left[leading_skipped:]
+            right = right[leading_skipped:]
+        i = 0
         if len(left) == len(right):
             for i in range(len(left)):
                 if left[-i] != right[-i]:
@@ -308,6 +311,21 @@
                 ]
                 left = left[:-i]
                 right = right[:-i]
+        shortest = min(left, right, key=lambda x: len(x))
+        lines = j = start = 0
+        if shortest.count("\n") >= DEFAULT_MAX_LINES:
+            # Keep only DEFAULT_MAX_LINES, usually 8, lines
+            if 10 < leading_skipped < 42:  # We didn't skip equal leading characters
+                start += leading_skipped - 10
+            for j, c in enumerate(shortest[start:], start=start - 1):
+                if c == "\n":
+                    lines += 1
+                if lines > DEFAULT_MAX_LINES:
+                    break
+        else:
+            j = len(max(left, right, key=lambda x: len(x)))
+        left = left[start : min(DEFAULT_MAX_CHARS, len(left), j)]
+        right = right[start : min(DEFAULT_MAX_CHARS, len(right), j)]
     keepends = True
     if left.isspace() or right.isspace():
         left = repr(str(left))

testing/test_assertion.py

@@ -417,6 +417,42 @@ def test_multiline_text_diff(self) -> None:
         assert "- eggs" in diff
         assert "+ spam" in diff
 
+    def test_long_text_diff_same_trailing(self) -> None:
+        left = "foo\nbar\n" * 100 + "spam\n" * 100 + "spam\n" * 160
+        right = "foo\nbar\n" * 100 + "eggs\n" * 100 + "spam\n" * 160
+        diff = callequal(left, right, verbose=0)
+        assert diff is not None
+        assert "- eggs" in diff
+        assert len(diff) == 19
+
+    def test_long_text_diff_different_trailing(self) -> None:
+        left = "foo\nbar\n" * 100 + "spam\n" * 100 + "spam\n" * 160
+        right = "foo\nbar\n" * 100 + "eggs\n" * 100 + "eggs\n" * 160
+        diff = callequal(left, right, verbose=0)
+        assert diff is not None
+        assert "- eggs" in diff
+        assert len(diff) == 18
+
+    def test_long_text_diff_short_trailing(self) -> None:
+        left = "foo\nbar\n" * 5 + "spam\n" * 5 + "spam\n" * 5
+        right = "foo\nbar\n" * 5 + "eggs\n" * 5 + "spam\n" * 5
+        diff = callequal(left, right, verbose=0)
+        assert diff is not None
+        assert "- eggs" in diff
+        assert len(diff) == 16
+
+    def test_long_text_diff_long_lines(self) -> None:
+        long_line1 = "foo" * 50 + "\n"
+        long_line2 = "bar" * 50 + "\n"
+        left = long_line1 * 3 + "spam\n" * 3 + long_line1 * 10
+        right = long_line1 * 3 + "eggs\n" * 3 + long_line2 * 10
+        diff = callequal(left, right, verbose=0)
+        assert diff is not None
+        assert "- eggs" in diff
+        assert "+ " + long_line1[:-1] in diff
+        assert "- " + long_line2[:-1] in diff
+        assert len(diff) == 20
+
     def test_bytes_diff_normal(self) -> None:
         """Check special handling for bytes diff (#5260)"""
         diff = callequal(b"spam", b"eggs")