Use code highlighting if pygments is installed (#6658)

* Use code highlighting if pygments is installed

* Use colorama constants instead of bare ascii codes

Could not find the exact equivalent of 'hl-reset' code using colorama
constants though.

* Refactor ASCII color handling into a fixture

* Revert back to using explicit color codes

* In Python 3.5 skip rest of tests that require ordered markup in colored output

src/_pytest/_code/code.py

@@ -1039,21 +1039,58 @@ class ReprEntry(TerminalRepr):
         self.reprfileloc = filelocrepr
         self.style = style
 
+    def _write_entry_lines(self, tw: TerminalWriter) -> None:
+        """Writes the source code portions of a list of traceback entries with syntax highlighting.
+
+        Usually entries are lines like these:
+
+            "     x = 1"
+            ">    assert x == 2"
+            "E    assert 1 == 2"
+
+        This function takes care of rendering the "source" portions of it (the lines without
+        the "E" prefix) using syntax highlighting, taking care to not highlighting the ">"
+        character, as doing so might break line continuations.
+        """
+
+        indent_size = 4
+
+        def is_fail(line):
+            return line.startswith("{}   ".format(FormattedExcinfo.fail_marker))
+
+        if not self.lines:
+            return
+
+        # separate indents and source lines that are not failures: we want to
+        # highlight the code but not the indentation, which may contain markers
+        # such as ">   assert 0"
+        indents = []
+        source_lines = []
+        for line in self.lines:
+            if not is_fail(line):
+                indents.append(line[:indent_size])
+                source_lines.append(line[indent_size:])
+
+        tw._write_source(source_lines, indents)
+
+        # failure lines are always completely red and bold
+        for line in (x for x in self.lines if is_fail(x)):
+            tw.line(line, bold=True, red=True)
+
     def toterminal(self, tw: TerminalWriter) -> None:
         if self.style == "short":
             assert self.reprfileloc is not None
             self.reprfileloc.toterminal(tw)
-            for line in self.lines:
-                red = line.startswith("E   ")
-                tw.line(line, bold=True, red=red)
+            self._write_entry_lines(tw)
             if self.reprlocals:
                 self.reprlocals.toterminal(tw, indent=" " * 8)
             return
+
         if self.reprfuncargs:
             self.reprfuncargs.toterminal(tw)
-        for line in self.lines:
-            red = line.startswith("E   ")
-            tw.line(line, bold=True, red=red)
+
+        self._write_entry_lines(tw)
+
         if self.reprlocals:
             tw.line("")
             self.reprlocals.toterminal(tw)

src/_pytest/_io/__init__.py

@@ -1,3 +1,39 @@
-# Reexport TerminalWriter from here instead of py, to make it easier to
-# extend or swap our own implementation in the future.
-from py.io import TerminalWriter as TerminalWriter  # noqa: F401
+from typing import List
+from typing import Sequence
+
+from py.io import TerminalWriter as BaseTerminalWriter  # noqa: F401
+
+
+class TerminalWriter(BaseTerminalWriter):
+    def _write_source(self, lines: List[str], indents: Sequence[str] = ()) -> None:
+        """Write lines of source code possibly highlighted.
+
+        Keeping this private for now because the API is clunky. We should discuss how
+        to evolve the terminal writer so we can have more precise color support, for example
+        being able to write part of a line in one color and the rest in another, and so on.
+        """
+        if indents and len(indents) != len(lines):
+            raise ValueError(
+                "indents size ({}) should have same size as lines ({})".format(
+                    len(indents), len(lines)
+                )
+            )
+        if not indents:
+            indents = [""] * len(lines)
+        source = "\n".join(lines)
+        new_lines = self._highlight(source).splitlines()
+        for indent, new_line in zip(indents, new_lines):
+            self.line(indent + new_line)
+
+    def _highlight(self, source):
+        """Highlight the given source code according to the "code_highlight" option"""
+        if not self.hasmarkup:
+            return source
+        try:
+            from pygments.formatters.terminal import TerminalFormatter
+            from pygments.lexers.python import PythonLexer
+            from pygments import highlight
+        except ImportError:
+            return source
+        else:
+            return highlight(source, PythonLexer(), TerminalFormatter(bg="dark"))