Use streaming in all assertion comparisons consumers

changelog/14523.improvement.rst

@@ -0,0 +1,27 @@
+Assertion explanations are now built lazily and the truncator stops
+the comparison helpers as soon as it has enough output, so comparing
+two large collections no longer builds the full diff in order to
+discard it. A focused micro-benchmark the worst case scenario
+(``set(range(500_000)) == set(range(1, 500_001))``) drops from ~2,200 ms
+to ~43 ms; but realistic test suite with mostly small diffs should be
+unchanged.
+
+String comparisons (the ``ndiff`` diff style) are bounded the same way:
+``difflib.ndiff`` computes the whole diff up front and cannot be
+short-circuited by streaming, so its inputs are capped to the truncation
+budget instead. A large ``str == str`` mismatch that previously spent
+seconds in ``ndiff`` is now near-instant. When truncating, the diff
+reflects a local alignment of the bounded prefix, which can differ from
+the full diff for wholly-dissimilar inputs; use ``-vv`` for the exact
+full diff.
+
+A dict comparison with many keys only on one side ("Left/Right contains
+N more items") no longer pretty-prints the whole extra subdict when it is
+going to be truncated: past the budget it shows the smallest few keys one
+per line instead. Small comparisons are unchanged (the compact block).
+
+The truncation footer no longer reports the hidden-line count
+(``...Full output truncated (N lines hidden), ...`` becomes
+``...Full output truncated, ...``); diff lines now carry a redundant
+``\x1b[0m`` reset prefix (invisible to terminals) so we can handle
+line one by one.

doc/en/example/reportingdemo.rst

@@ -148,7 +148,7 @@ Here is a nice run of several failures and how ``pytest`` presents things:
     E           1
     E           1...
     E
-    E         ...Full output truncated (7 lines hidden), use '-vv' to show
+    E         ...Full output truncated, use '-vv' to show
 
     failure_demo.py:62: AssertionError
     _________________ TestSpecialisedExplanations.test_eq_list _________________

doc/en/how-to/output.rst

@@ -172,7 +172,7 @@ Now we can increase pytest's verbosity:
     E               'banana',
     E               'apple',...
     E
-    E         ...Full output truncated (7 lines hidden), use '-vv' to show
+    E         ...Full output truncated, use '-vv' to show
 
     test_verbosity_example.py:8: AssertionError
     ____________________________ test_numbers_fail _____________________________
@@ -190,7 +190,7 @@ Now we can increase pytest's verbosity:
     E         {'10': 10, '20': 20, '30': 30, '40': 40}
     E         ...
     E
-    E         ...Full output truncated (16 lines hidden), use '-vv' to show
+    E         ...Full output truncated, use '-vv' to show
 
     test_verbosity_example.py:14: AssertionError
     ___________________________ test_long_text_fail ____________________________