Add meta test for new diff logic (#16211)

Follow up to #16112

Author: hauntsaninja

mypy/test/helpers.py

@@ -8,7 +8,7 @@
 import shutil
 import sys
 import time
-from typing import Any, Callable, Iterable, Iterator, Pattern
+from typing import IO, Any, Callable, Iterable, Iterator, Pattern
 
 # Exporting Suite as alias to TestCase for backwards compatibility
 # TODO: avoid aliasing - import and subclass TestCase directly
@@ -70,7 +70,12 @@ def diff_ranges(
 
 
 def render_diff_range(
-    ranges: list[tuple[int, int]], content: list[str], colour: str | None = None
+    ranges: list[tuple[int, int]],
+    content: list[str],
+    *,
+    colour: str | None = None,
+    output: IO[str] = sys.stderr,
+    indent: int = 2,
 ) -> None:
     for i, line_range in enumerate(ranges):
         is_matching = i % 2 == 1
@@ -83,20 +88,20 @@ def render_diff_range(
                 and j < len(lines) - 3
             ):
                 if j == 3:
-                    sys.stderr.write("  ...\n")
+                    output.write(" " * indent + "...\n")
                 continue
 
             if not is_matching and colour:
-                sys.stderr.write(colour)
+                output.write(colour)
 
-            sys.stderr.write("  " + line)
+            output.write(" " * indent + line)
 
             if not is_matching:
                 if colour:
-                    sys.stderr.write("\033[0m")
-                sys.stderr.write(" (diff)")
+                    output.write("\033[0m")
+                output.write(" (diff)")
 
-            sys.stderr.write("\n")
+            output.write("\n")
 
 
 def assert_string_arrays_equal(expected: list[str], actual: list[str], msg: str) -> None:
@@ -129,7 +134,7 @@ def assert_string_arrays_equal(expected: list[str], actual: list[str], msg: str)
 
         sys.stderr.write(
             "Update the test output using --update-data -n0 "
-            "(you can additionally use the -k selector to update only specific tests)"
+            "(you can additionally use the -k selector to update only specific tests)\n"
         )
         pytest.fail(msg, pytrace=False)
 

mypy/test/meta/test_diff_helper.py

@@ -0,0 +1,47 @@
+import io
+
+from mypy.test.helpers import Suite, diff_ranges, render_diff_range
+
+
+class DiffHelperSuite(Suite):
+    def test_render_diff_range(self) -> None:
+        expected = ["hello", "world"]
+        actual = ["goodbye", "world"]
+
+        expected_ranges, actual_ranges = diff_ranges(expected, actual)
+
+        output = io.StringIO()
+        render_diff_range(expected_ranges, expected, output=output)
+        assert output.getvalue() == "  hello (diff)\n  world\n"
+        output = io.StringIO()
+        render_diff_range(actual_ranges, actual, output=output)
+        assert output.getvalue() == "  goodbye (diff)\n  world\n"
+
+        expected = ["a", "b", "c", "d", "e", "f", "g", "h", "circle", "i", "j"]
+        actual = ["a", "b", "c", "d", "e", "f", "g", "h", "square", "i", "j"]
+
+        expected_ranges, actual_ranges = diff_ranges(expected, actual)
+
+        output = io.StringIO()
+        render_diff_range(expected_ranges, expected, output=output, indent=0)
+        assert output.getvalue() == "a\nb\nc\n...\nf\ng\nh\ncircle (diff)\ni\nj\n"
+        output = io.StringIO()
+        render_diff_range(actual_ranges, actual, output=output, indent=0)
+        assert output.getvalue() == "a\nb\nc\n...\nf\ng\nh\nsquare (diff)\ni\nj\n"
+
+    def test_diff_ranges(self) -> None:
+        a = ["hello", "world"]
+        b = ["hello", "world"]
+
+        assert diff_ranges(a, b) == (
+            [(0, 0), (0, 2), (2, 2), (2, 2)],
+            [(0, 0), (0, 2), (2, 2), (2, 2)],
+        )
+
+        a = ["hello", "world"]
+        b = ["goodbye", "world"]
+
+        assert diff_ranges(a, b) == (
+            [(0, 1), (1, 2), (2, 2), (2, 2)],
+            [(0, 1), (1, 2), (2, 2), (2, 2)],
+        )