Also highlight comparisons between strings

Fixes #13175

Author: The-Compiler

changelog/13175.bugfix.rst

@@ -0,0 +1 @@
+The diff is now also highlighted correctly when comparing two strings.

src/_pytest/assertion/util.py

@@ -43,6 +43,14 @@ def __call__(self, source: str, lexer: Literal["diff", "python"] = "python") ->
         """Apply highlighting to the given source."""
 
 
+def dummy_highlighter(source: str, lexer: Literal["diff", "python"] = "python") -> str:
+    """Dummy highlighter that returns the text unprocessed.
+
+    Needed for _notin_text, as the diff gets post-processed to only show the "+" part.
+    """
+    return source
+
+
 def format_explanation(explanation: str) -> str:
     r"""Format an explanation.
 
@@ -242,7 +250,7 @@ def _compare_eq_any(
 ) -> list[str]:
     explanation = []
     if istext(left) and istext(right):
-        explanation = _diff_text(left, right, verbose)
+        explanation = _diff_text(left, right, highlighter, verbose)
     else:
         from _pytest.python_api import ApproxBase
 
@@ -274,7 +282,9 @@ def _compare_eq_any(
     return explanation
 
 
-def _diff_text(left: str, right: str, verbose: int = 0) -> list[str]:
+def _diff_text(
+    left: str, right: str, highlighter: _HighlightFunc, verbose: int = 0
+) -> list[str]:
     """Return the explanation for the diff between text.
 
     Unless --verbose is used this will skip leading and trailing
@@ -315,10 +325,15 @@ def _diff_text(left: str, right: str, verbose: int = 0) -> list[str]:
         explanation += ["Strings contain only whitespace, escaping them using repr()"]
     # "right" is the expected base against which we compare "left",
     # see https://github.com/pytest-dev/pytest/issues/3333
-    explanation += [
-        line.strip("\n")
-        for line in ndiff(right.splitlines(keepends), left.splitlines(keepends))
-    ]
+    explanation.extend(
+        highlighter(
+            "\n".join(
+                line.strip("\n")
+                for line in ndiff(right.splitlines(keepends), left.splitlines(keepends))
+            ),
+            lexer="diff",
+        ).splitlines()
+    )
     return explanation
 
 
@@ -586,7 +601,7 @@ def _notin_text(term: str, text: str, verbose: int = 0) -> list[str]:
     head = text[:index]
     tail = text[index + len(term) :]
     correct_text = head + tail
-    diff = _diff_text(text, correct_text, verbose)
+    diff = _diff_text(text, correct_text, dummy_highlighter, verbose)
     newdiff = [f"{saferepr(term, maxsize=42)} is contained here:"]
     for line in diff:
         if line.startswith("Skipping"):

testing/test_assertion.py

@@ -2019,6 +2019,16 @@ def test():
                 "{bold}{red}E         {light-green}+     'number-is-5': 5,{hl-reset}{endline}{reset}",
             ],
         ),
+        (
+            """
+            def test():
+                assert "abcd" == "abce"
+            """,
+            [
+                "{bold}{red}E         {reset}{light-red}- abce{hl-reset}{endline}{reset}",
+                "{bold}{red}E         {light-green}+ abcd{hl-reset}{endline}{reset}",
+            ],
+        ),
     ),
 )
 def test_comparisons_handle_colors(