diff --git a/pyformat.py b/pyformat.py
index de5e33b..c916891 100755
--- a/pyformat.py
+++ b/pyformat.py
@@ -31,6 +31,7 @@
 import io
 import signal
 import sys
+from typing import Tuple
 
 import autoflake
 import autopep8
@@ -76,16 +77,43 @@ def format_code(source, aggressive=False, apply_config=False, filename='',
     return formatted_source
 
 
+def detect_io_encoding(input_file: io.BytesIO, limit_byte_check=-1):
+    """Return file encoding."""
+    try:
+        from lib2to3.pgen2 import tokenize as lib2to3_tokenize
+        encoding: str = lib2to3_tokenize.detect_encoding(input_file.readline)[
+            0]
+
+        input_file.read(limit_byte_check).decode(encoding)
+
+        return encoding
+    except (LookupError, SyntaxError, UnicodeDecodeError):
+        return 'latin-1'
+
+
+def read_file(filename: str) -> Tuple[str, str]:
+    """Read file from filesystem or from stdin when `-` is given."""
+
+    if is_stdin(filename):
+        data = sys.stdin.buffer.read()
+    else:
+        with open(filename, 'rb') as fp:
+            data = fp.read()
+    input_file = io.BytesIO(data)
+    encoding = detect_io_encoding(input_file)
+    return data.decode(encoding), encoding
+
+
+def is_stdin(filename: str):
+    return filename == '-'
+
+
 def format_file(filename, args, standard_out):
     """Run format_code() on a file.
 
     Return True if the new formatting differs from the original.
-
     """
-    encoding = autopep8.detect_encoding(filename)
-    with autopep8.open_with_encoding(filename,
-                                     encoding=encoding) as input_file:
-        source = input_file.read()
+    source, encoding = read_file(filename)
 
     if not source:
         return False
@@ -98,6 +126,12 @@ def format_file(filename, args, standard_out):
         remove_all_unused_imports=args.remove_all_unused_imports,
         remove_unused_variables=args.remove_unused_variables)
 
+    # Always write to stdout (even when no changes were made) when working with
+    # in-place stdin. This is what most tools (editors) expect.
+    if args.in_place and is_stdin(filename):
+        standard_out.write(formatted_source)
+        return True
+
     if source != formatted_source:
         if args.in_place:
             with autopep8.open_with_encoding(filename, mode='w',
@@ -142,7 +176,6 @@ def format_multiple_files(filenames, args, standard_out, standard_error):
     """Format files and return booleans (any_changes, any_errors).
 
     Optionally format files recursively.
-
     """
     filenames = autopep8.find_files(list(filenames),
                                     args.recursive,
@@ -211,7 +244,6 @@ def _main(argv, standard_out, standard_error):
     """Internal main entry point.
 
     Return exit status. 0 means no error.
-
     """
     args = parse_args(argv)
 
diff --git a/test_pyformat.py b/test_pyformat.py
index dbe5fdb..6337db9 100755
--- a/test_pyformat.py
+++ b/test_pyformat.py
@@ -13,6 +13,7 @@
 import sys
 import tempfile
 import unittest
+from unittest import mock
 
 import pyformat
 
@@ -144,6 +145,58 @@ def test_format_multiple_files_with_nonexistent_file_and_verbose(self):
 
 class TestSystem(unittest.TestCase):
 
+    def test_diff_stdin(self):
+        input_b = b'''\
+import os
+x = "abc"
+'''
+        stdin = io.TextIOWrapper(io.BytesIO(input_b), 'UTF-8')
+        with mock.patch.object(sys, 'stdin', stdin):
+            output_file = io.StringIO()
+            pyformat._main(argv=['my_fake_program', '-'],
+                           standard_out=output_file,
+                           standard_error=None)
+            self.assertEqual('''\
+@@ -1,2 +1,2 @@
+ import os
+-x = "abc"
++x = 'abc'
+''', '\n'.join(output_file.getvalue().split('\n')[2:]))
+
+    def test_in_place_stdin(self):
+        input_b = b'''\
+import os
+x = "abc"
+'''
+        stdin = io.TextIOWrapper(io.BytesIO(input_b), 'UTF-8')
+        with mock.patch.object(sys, 'stdin', stdin):
+            output_file = io.StringIO()
+            pyformat._main(argv=['my_fake_program', '--in-place', '-'],
+                           standard_out=output_file,
+                           standard_error=None)
+            self.assertEqual('''\
+import os
+x = 'abc'
+''', '\n'.join(output_file.getvalue().split('\n')))
+
+
+    def test_in_place_stdin_no_change(self):
+        input_b = b'''\
+import os
+x = 'abc'
+'''
+        stdin = io.TextIOWrapper(io.BytesIO(input_b), 'UTF-8')
+        with mock.patch.object(sys, 'stdin', stdin):
+            output_file = io.StringIO()
+            pyformat._main(argv=['my_fake_program', '--in-place', '-'],
+                           standard_out=output_file,
+                           standard_error=None)
+            self.assertEqual('''\
+import os
+x = 'abc'
+''', '\n'.join(output_file.getvalue().split('\n')))
+
+
     def test_diff(self):
         with temporary_file('''\
 import os