[fix] Truncate only if truncating means a smaller output

Also fixes the dislayed line hidden message

Closes #6267

Author: Pierre-Sassoulas

AUTHORS

@@ -276,6 +276,7 @@ Paweł Adamczak
 Pedro Algarvio
 Petter Strandmark
 Philipp Loose
+Pierre Sassoulas
 Pieter Mulder
 Piotr Banaszkiewicz
 Piotr Helm

changelog/6267.feature.rst

@@ -0,0 +1,2 @@
+The full output of a test is no longer truncated if the truncation message would be longer than
+the hidden text. The line number shown has also been fixed.

src/_pytest/assertion/truncate.py

@@ -38,45 +38,61 @@ def _truncate_explanation(
     """Truncate given list of strings that makes up the assertion explanation.
 
     Truncates to either 8 lines, or 640 characters - whichever the input reaches
-    first. The remaining lines will be replaced by a usage message.
+    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))
-    if len(input_lines) <= max_lines and input_char_count <= max_chars:
+    # The length of the truncation explanation depends on the number of line
+    # removed but is at least 67 characters:
+    # The real value is
+    # 64 (for the base message:
+    # '...\n...Full output truncated (1 line hidden), use '-vv' to show")'
+    # )
+    # + 1 (for plural)
+    # + int(math.log10(len(input_lines) - max_lines)) (number of hidden line)
+    # + 3 for the '...' added to the truncated line
+    # But if there's more than 100 lines it's very likely that we're going to
+    # truncate, so we don't need the exact value using log10.
+    tolerable_max_chars = (
+        max_chars + 70  # 64 + 1 (for plural) + 2 (for '99') + 3 for '...'
+    )
+    # The truncation explanation add two lines to the output
+    tolerable_max_lines = max_lines + 2
+    if (
+        len(input_lines) <= tolerable_max_lines
+        and input_char_count <= tolerable_max_chars
+    ):
         return input_lines
-
-    # Truncate first to max_lines, and then truncate to max_chars if max_chars
-    # is exceeded.
+    # Truncate first to max_lines, and then truncate to max_chars if necessary
     truncated_explanation = input_lines[:max_lines]
-    truncated_explanation = _truncate_by_char_count(truncated_explanation, max_chars)
-
-    # Add ellipsis to final line
-    truncated_explanation[-1] = truncated_explanation[-1] + "..."
+    # We reevaluate the need to truncate following removal of some lines
+    if len("".join(input_lines)) > tolerable_max_chars:
+        truncated_explanation = _truncate_by_char_count(
+            truncated_explanation, max_chars
+        )
 
-    # Append useful message to explanation
     truncated_line_count = len(input_lines) - len(truncated_explanation)
-    truncated_line_count += 1  # Account for the part-truncated final line
-    truncated_explanation.extend(
-        [
-            "",  # Line break
-            f"...Full output truncated ({truncated_line_count} line"
-            f"{'' if truncated_line_count == 1 else 's'} hidden), {USAGE_MSG}",
-        ]
-    )
-    return truncated_explanation
+    if truncated_explanation[-1]:
+        # Add ellipsis and take into account part-truncated final line
+        truncated_explanation[-1] = truncated_explanation[-1] + "..."
+        truncated_line_count += 1
+    else:
+        # Add proper ellipsis when we were able to fit a full line exactly
+        truncated_explanation[-1] = "..."
+    return truncated_explanation + [
+        "",
+        f"...Full output truncated ({truncated_line_count} line"
+        f"{'' if truncated_line_count == 1 else 's'} hidden), {USAGE_MSG}",
+    ]
 
 
 def _truncate_by_char_count(input_lines: List[str], max_chars: int) -> List[str]:
-    # Check if truncation required
-    if len("".join(input_lines)) <= max_chars:
-        return input_lines
-
     # Find point at which input length exceeds total allowed length
     iterated_char_count = 0
     for iterated_index, input_line in enumerate(input_lines):

testing/test_assertion.py

@@ -807,9 +807,9 @@ def test_dataclasses(self, pytester: Pytester) -> None:
                 "E         ['field_b']",
                 "E         ",
                 "E         Drill down into differing attribute field_b:",
-                "E           field_b: 'b' != 'c'...",
-                "E         ",
-                "E         ...Full output truncated (3 lines hidden), use '-vv' to show",
+                "E           field_b: 'b' != 'c'",
+                "E           - c",
+                "E           + b",
             ],
             consecutive=True,
         )
@@ -1188,10 +1188,11 @@ def test_doesnt_truncate_at_when_input_is_5_lines_and_LT_max_chars(self) -> None
     def test_truncates_at_8_lines_when_given_list_of_empty_strings(self) -> None:
         expl = ["" for x in range(50)]
         result = truncate._truncate_explanation(expl, max_lines=8, max_chars=100)
+        assert len(result) != len(expl)
         assert result != expl
         assert len(result) == 8 + self.LINES_IN_TRUNCATION_MSG
         assert "Full output truncated" in result[-1]
-        assert "43 lines hidden" in result[-1]
+        assert "42 lines hidden" in result[-1]
         last_line_before_trunc_msg = result[-self.LINES_IN_TRUNCATION_MSG - 1]
         assert last_line_before_trunc_msg.endswith("...")
 
@@ -1205,13 +1206,34 @@ def test_truncates_at_8_lines_when_first_8_lines_are_LT_max_chars(self) -> None:
         last_line_before_trunc_msg = result[-self.LINES_IN_TRUNCATION_MSG - 1]
         assert last_line_before_trunc_msg.endswith("...")
 
+    def test_truncates_at_8_lines_when_there_is_one_line_to_remove(self) -> None:
+        """The number of line in the result is 9, the same number as if we truncated."""
+        expl = ["a" for x in range(9)]
+        result = truncate._truncate_explanation(expl, max_lines=8, max_chars=8 * 80)
+        assert result == expl
+        assert "truncated" not in result[-1]
+
+    def test_truncates_edgecase_when_truncation_message_makes_the_result_longer_for_chars(
+        self,
+    ) -> None:
+        expl = ["a" * 10 for x in range(2)]
+        result = truncate._truncate_explanation(expl, max_lines=10, max_chars=10)
+        assert result == ["aaaaaaaaaa", "aaaaaaaaaa"]
+
+    def test_truncates_edgecase_when_truncation_message_makes_the_result_longer_for_lines(
+        self,
+    ) -> None:
+        expl = ["a" * 10 for x in range(2)]
+        result = truncate._truncate_explanation(expl, max_lines=1, max_chars=100)
+        assert result == ["aaaaaaaaaa", "aaaaaaaaaa"]
+
     def test_truncates_at_8_lines_when_first_8_lines_are_EQ_max_chars(self) -> None:
         expl = ["a" * 80 for x in range(16)]
         result = truncate._truncate_explanation(expl, max_lines=8, max_chars=8 * 80)
         assert result != expl
-        assert len(result) == 8 + self.LINES_IN_TRUNCATION_MSG
+        assert len(result) == 16 - 8 + self.LINES_IN_TRUNCATION_MSG
         assert "Full output truncated" in result[-1]
-        assert "9 lines hidden" in result[-1]
+        assert "8 lines hidden" in result[-1]
         last_line_before_trunc_msg = result[-self.LINES_IN_TRUNCATION_MSG - 1]
         assert last_line_before_trunc_msg.endswith("...")