Make pygments mandatory and fix string highlighting

changelog/13175.bugfix.rst

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

changelog/7683.improvement.rst

@@ -0,0 +1 @@
+The formerly optional ``pygments`` dependency is now required, causing output always to be source-highlighted (unless disabled via the ``--code-highlight=no`` CLI option).

pyproject.toml

@@ -51,14 +51,14 @@ dependencies = [
     "iniconfig",
     "packaging",
     "pluggy>=1.5,<2",
+    "pygments>=2.7.2",
     "tomli>=1; python_version<'3.11'",
 ]
 optional-dependencies.dev = [
     "argcomplete",
     "attrs>=19.2",
     "hypothesis>=3.56",
     "mock",
-    "pygments>=2.7.2",
     "requests",
     "setuptools",
     "xmlschema",

src/_pytest/_io/terminalwriter.py

@@ -9,17 +9,17 @@
 from typing import final
 from typing import Literal
 from typing import TextIO
-from typing import TYPE_CHECKING
+
+import pygments
+from pygments.formatters.terminal import TerminalFormatter
+from pygments.lexer import Lexer
+from pygments.lexers.diff import DiffLexer
+from pygments.lexers.python import PythonLexer
 
 from ..compat import assert_never
 from .wcwidth import wcswidth
 
 
-if TYPE_CHECKING:
-    from pygments.formatter import Formatter
-    from pygments.lexer import Lexer
-
-
 # This code was initially copied from py 1.8.1, file _io/terminalwriter.py.
 
 
@@ -201,37 +201,22 @@ def _write_source(self, lines: Sequence[str], indents: Sequence[str] = ()) -> No
         for indent, new_line in zip(indents, new_lines):
             self.line(indent + new_line)
 
-    def _get_pygments_lexer(self, lexer: Literal["python", "diff"]) -> Lexer | None:
-        try:
-            if lexer == "python":
-                from pygments.lexers.python import PythonLexer
-
-                return PythonLexer()
-            elif lexer == "diff":
-                from pygments.lexers.diff import DiffLexer
-
-                return DiffLexer()
-            else:
-                assert_never(lexer)
-        except ModuleNotFoundError:
-            return None
-
-    def _get_pygments_formatter(self) -> Formatter | None:
-        try:
-            import pygments.util
-        except ModuleNotFoundError:
-            return None
+    def _get_pygments_lexer(self, lexer: Literal["python", "diff"]) -> Lexer:
+        if lexer == "python":
+            return PythonLexer()
+        elif lexer == "diff":
+            return DiffLexer()
+        else:
+            assert_never(lexer)
 
+    def _get_pygments_formatter(self) -> TerminalFormatter:
         from _pytest.config.exceptions import UsageError
 
         theme = os.getenv("PYTEST_THEME")
         theme_mode = os.getenv("PYTEST_THEME_MODE", "dark")
 
         try:
-            from pygments.formatters.terminal import TerminalFormatter
-
             return TerminalFormatter(bg=theme_mode, style=theme)
-
         except pygments.util.ClassNotFound as e:
             raise UsageError(
                 f"PYTEST_THEME environment variable has an invalid value: '{theme}'. "
@@ -251,16 +236,11 @@ def _highlight(
             return source
 
         pygments_lexer = self._get_pygments_lexer(lexer)
-        if pygments_lexer is None:
-            return source
-
         pygments_formatter = self._get_pygments_formatter()
-        if pygments_formatter is None:
-            return source
-
-        from pygments import highlight
 
-        highlighted: str = highlight(source, pygments_lexer, pygments_formatter)
+        highlighted: str = pygments.highlight(
+            source, pygments_lexer, pygments_formatter
+        )
         # pygments terminal formatter may add a newline when there wasn't one.
         # We don't want this, remove.
         if highlighted[-1] == "\n" and source[-1] != "\n":

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(