Add piping from stdin to stdout with a - (#25)

Being able to format code by piping it through the formatter makes it much easier to integrate with tools like google/vim-codefmt or Chiel92/vim-autoformat.

black.py

@@ -74,7 +74,9 @@ class CannotSplit(Exception):
 @click.argument(
     'src',
     nargs=-1,
-    type=click.Path(exists=True, file_okay=True, dir_okay=True, readable=True),
+    type=click.Path(
+        exists=True, file_okay=True, dir_okay=True, readable=True, allow_dash=True
+    ),
 )
 @click.pass_context
 def main(
@@ -89,6 +91,8 @@ def main(
         elif p.is_file():
             # if a file was explicitly given, we don't care about its extension
             sources.append(p)
+        elif s == '-':
+            sources.append(Path('-'))
         else:
             err(f'invalid path: {s}')
     if len(sources) == 0:
@@ -97,9 +101,12 @@ def main(
         p = sources[0]
         report = Report()
         try:
-            changed = format_file_in_place(
-                p, line_length=line_length, fast=fast, write_back=not check
-            )
+            if not p.is_file() and str(p) == '-':
+                changed = format_stdin_to_stdout(line_length=line_length, fast=fast)
+            else:
+                changed = format_file_in_place(
+                    p, line_length=line_length, fast=fast, write_back=not check
+                )
             report.done(p, changed)
         except Exception as exc:
             report.failed(p, str(exc))
@@ -156,34 +163,50 @@ def format_file_in_place(
     src: Path, line_length: int, fast: bool, write_back: bool = False
 ) -> bool:
     """Format the file and rewrite if changed. Return True if changed."""
+    with tokenize.open(src) as src_buffer:
+        src_contents = src_buffer.read()
     try:
-        contents, encoding = format_file(src, line_length=line_length, fast=fast)
+        contents = format_file_contents(
+            src_contents, line_length=line_length, fast=fast
+        )
     except NothingChanged:
         return False
 
     if write_back:
-        with open(src, "w", encoding=encoding) as f:
+        with open(src, "w", encoding=src_buffer.encoding) as f:
             f.write(contents)
     return True
 
 
-def format_file(
-    src: Path, line_length: int, fast: bool
-) -> Tuple[FileContent, Encoding]:
+def format_stdin_to_stdout(line_length: int, fast: bool) -> bool:
+    """Format file on stdin and pipe output to stdout. Return True if changed."""
+    contents = sys.stdin.read()
+    try:
+        contents = format_file_contents(contents, line_length=line_length, fast=fast)
+        return True
+
+    except NothingChanged:
+        return False
+
+    finally:
+        sys.stdout.write(contents)
+
+
+def format_file_contents(
+    src_contents: str, line_length: int, fast: bool
+) -> FileContent:
     """Reformats a file and returns its contents and encoding."""
-    with tokenize.open(src) as src_buffer:
-        src_contents = src_buffer.read()
     if src_contents.strip() == '':
-        raise NothingChanged(src)
+        raise NothingChanged
 
     dst_contents = format_str(src_contents, line_length=line_length)
     if src_contents == dst_contents:
-        raise NothingChanged(src)
+        raise NothingChanged
 
     if not fast:
         assert_equivalent(src_contents, dst_contents)
         assert_stable(src_contents, dst_contents, line_length=line_length)
-    return dst_contents, src_buffer.encoding
+    return dst_contents
 
 
 def format_str(src_contents: str, line_length: int) -> FileContent:

tests/test_black.py

@@ -1,6 +1,8 @@
 #!/usr/bin/env python3
 from functools import partial
+from io import StringIO
 from pathlib import Path
+import sys
 from typing import Any, List, Tuple
 import unittest
 from unittest.mock import patch
@@ -10,7 +12,7 @@
 import black
 
 ll = 88
-ff = partial(black.format_file, line_length=ll, fast=True)
+ff = partial(black.format_file_in_place, line_length=ll, fast=True)
 fs = partial(black.format_str, line_length=ll)
 THIS_FILE = Path(__file__)
 THIS_DIR = THIS_FILE.parent
@@ -70,8 +72,7 @@ def test_self(self) -> None:
         self.assertFormatEqual(expected, actual)
         black.assert_equivalent(source, actual)
         black.assert_stable(source, actual, line_length=ll)
-        with self.assertRaises(black.NothingChanged):
-            ff(THIS_FILE)
+        self.assertFalse(ff(THIS_FILE))
 
     @patch("black.dump_to_file", dump_to_stderr)
     def test_black(self) -> None:
@@ -80,8 +81,22 @@ def test_black(self) -> None:
         self.assertFormatEqual(expected, actual)
         black.assert_equivalent(source, actual)
         black.assert_stable(source, actual, line_length=ll)
-        with self.assertRaises(black.NothingChanged):
-            ff(THIS_FILE)
+        self.assertFalse(ff(THIS_DIR / '..' / 'black.py'))
+
+    def test_piping(self) -> None:
+        source, expected = read_data('../black')
+        hold_stdin, hold_stdout = sys.stdin, sys.stdout
+        try:
+            sys.stdin, sys.stdout = StringIO(source), StringIO()
+            sys.stdin.name = '<stdin>'
+            black.format_stdin_to_stdout(line_length=ll, fast=True)
+            sys.stdout.seek(0)
+            actual = sys.stdout.read()
+        finally:
+            sys.stdin, sys.stdout = hold_stdin, hold_stdout
+        self.assertFormatEqual(expected, actual)
+        black.assert_equivalent(source, actual)
+        black.assert_stable(source, actual, line_length=ll)
 
     @patch("black.dump_to_file", dump_to_stderr)
     def test_setup(self) -> None:
@@ -90,8 +105,7 @@ def test_setup(self) -> None:
         self.assertFormatEqual(expected, actual)
         black.assert_equivalent(source, actual)
         black.assert_stable(source, actual, line_length=ll)
-        with self.assertRaises(black.NothingChanged):
-            ff(THIS_FILE)
+        self.assertFalse(ff(THIS_DIR / '..' / 'setup.py'))
 
     @patch("black.dump_to_file", dump_to_stderr)
     def test_function(self) -> None: