Add the -d / --stdout option. Fixes #164.

This still needs refining. Currently it prints out every file which was
changed by reformatting modified lines. We probably want to only print
out a single file.

src/darker/__main__.py

@@ -168,6 +168,21 @@ def print_diff(path: Path, old: TextDocument, new: TextDocument) -> None:
         print(diff)
 
 
+def print_source(new: TextDocument) -> None:
+    """Print the reformatted Python source code"""
+    if sys.stdout.isatty():
+        try:
+            from pygments import highlight
+            from pygments.formatters import TerminalFormatter
+            from pygments.lexers.python import PythonLexer
+        except ImportError:
+            print(new.string)
+        else:
+            print(highlight(new.string, PythonLexer(), TerminalFormatter()))
+    else:
+        print(new.string)
+
+
 def main(argv: List[str] = None) -> int:
     """Parse the command line and reformat and optionally lint each source file
 
@@ -233,7 +248,7 @@ def main(argv: List[str] = None) -> int:
     failures_on_modified_lines = False
 
     revrange = RevisionRange.parse(args.revision)
-    write_modified_files = not args.check and not args.diff
+    write_modified_files = not args.check and not args.diff and not args.stdout
     if revrange.rev2 != WORKTREE and write_modified_files:
         raise ArgumentError(
             Action(["-r", "--revision"], "revision"),
@@ -247,6 +262,8 @@ def main(argv: List[str] = None) -> int:
         failures_on_modified_lines = True
         if args.diff:
             print_diff(path, old, new)
+        elif args.stdout:
+            print_source(new)
         if write_modified_files:
             modify_file(path, new)
     if run_linters(args.lint, git_root, changed_files, revrange):

src/darker/command_line.py

@@ -32,6 +32,7 @@ def add_arg(help_text: Optional[Text], *name_or_flags: Text, **kwargs: Any) -> N
     add_arg(hlp.REVISION, "-r", "--revision", default="HEAD")
     add_arg(hlp.DIFF, "--diff", action="store_true")
     add_arg(hlp.CHECK, "--check", action="store_true")
+    add_arg(hlp.STDOUT, "-d", "--stdout", action="store_true")
     add_arg(hlp.ISORT, "-i", "--isort", action="store_true")
     add_arg(hlp.LINT, "-L", "--lint", action="append", metavar="CMD", default=[])
     add_arg(hlp.CONFIG, "-c", "--config", metavar="PATH")

src/darker/help.py

@@ -40,6 +40,11 @@
     " reformatted."
 )
 
+STDOUT = (
+    "Force complete reformatted output to stdout, instead of in-place. Only valid if"
+    " there's just one file to reformat."
+)
+
 ISORT_PARTS = ["Also sort imports using the `isort` package"]
 if not isort:
     ISORT_PARTS.append(f". {ISORT_INSTRUCTION} to enable usage of this option.")